AMC 8 Daily Practice - Consecutive Reduction

Complete problem set with solutions and individual problem pages

Problem 1 Easy

What is the value of 100\div(100\div99)\div(99\div98) \div\dots \div(3\div2)?

  • A.

    1

  • B.

    2

  • C.

    \frac{2}{3}

  • D.

    99

  • E.

    100

Answer:B

We observe that this problem contains repeated numbers and division symbols, so we expand the parentheses and rewrite the original expression as:

100 \div (100 \div 99) \div (99 \div 98) \div (98 \div 97) \div \dots \div (3 \div 2)=100 \div 100 \times 99 \div 99 \times 98 \div 98 \times 97 \div \dots \div 3 \times 2

We notice that consecutive pairs \not{100} \div\not{100}\times\not{99} \div \not{99} \times\not{98} \div \not{98} \times \dots \div\not{3} \times 2 cancel out completely.

Therefore, the final result of the original expression is \boxed{2}.

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Problem 2 Easy

What is the value of 1 \frac{1}{2} \times 1 \frac{1}{3} \times 1 \frac{1}{4} \times 1 \frac{1}{5} \times 1 \frac{1}{6} \times 1 \frac{1}{7} \times 1 \frac{1}{8} \times 1 \frac{1}{9}?

  • A.

    1

  • B.

    \frac{1}{5}

  • C.

    5

  • D.

    \frac{5}{9}

  • E.

    10

Answer:C

The problem involves mixed numbers.

We first convert each mixed number to an improper fraction:

1\frac{1}{2} \times 1\frac{1}{3} \times 1\frac{1}{4} \times 1\frac{1}{5} \times 1\frac{1}{6} \times 1\frac{1}{7} \times 1\frac{1}{8} \times 1\frac{1}{9}= \frac{3}{2} \times \frac{4}{3} \times \frac{5}{4} \times \frac{6}{5} \times \frac{7}{6} \times \frac{8}{7} \times \frac{9}{8} \times \frac{10}{9}= \frac{\not{3}}{2} \times \frac{\not{4}}{\not{3}} \times \frac{\not{5}}{\not{4}} \times \frac{\not{6}}{\not{5}} \times \frac{\not{7}}{\not{6}} \times \frac{\not{8}}{\not{7}} \times \frac{\not{9}}{\not{8}} \times \frac{10}{\not{9}}=\frac{1}{2} \times \frac{10}{1} = 5

Final result:\boxed{5}

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Problem 3 Easy

What is the value of \frac{1}{2} \times\left(1-\frac{1}{3}\right) \times\left(1-\frac{1}{4}\right) \times \cdots \times\left(1-\frac{1}{100}\right)?

  • A.

    \frac{1}{100}

  • B.

    \frac{1}{2}

  • C.

    \frac{1}{3}

  • D.

    \frac{1}{4}

  • E.

    \frac{1}{99}

Answer:A

First, we calculate the value inside each parenthesis, so the original expression can be rewritten as: \frac{1}{2} \times \left(1 - \frac{1}{3}\right) \times \left(1 - \frac{1}{4}\right) \times \cdots \times \left(1 - \frac{1}{100}\right)=\frac{1}{2} \times \frac{2}{3} \times \frac{3}{4} \times \cdots \times \frac{99}{100}

We observe a telescoping pattern where the denominator of each fraction cancels with the numerator of the subsequent fraction: \frac{1}{\not{2}} \times \frac{\not{2}}{\not{3}} \times \frac{\not{3}}{\not{4}} \times \cdots \times \frac{\not{99}}{100}

After complete cancellation of adjacent terms, only the first numerator and the last denominator remain: = \frac{1}{100}

Final result: \boxed{\frac{1}{100}}

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Problem 4 Easy

What id the value of \left(1-\frac{1}{2}\right) \times\left(2-\frac{2}{3}\right) \times\left(3-\frac{3}{4}\right) \times \cdots \times\left(8-\frac{8}{9}\right) \times\left(9-\frac{9}{10}\right)?

  • A.

    \frac{1}{2}

  • B.

    \frac{1}{3}

  • C.

    \frac{1}{10}

  • D.

    362880

  • E.

    36288

Answer:E

 Observing the expression, we notice repeated numbers in each parenthesis.

Factoring out these common terms, we can rewrite the expression as:   1 \times\left(1-\frac{1}{2}\right) \times 2 \times \left(1-\frac{1}{3}\right) \times 3 \times \left(1-\frac{1}{4}\right) \times \cdots \times 8 \times \left(1-\frac{1}{9}\right) \times 9 \times \left(1-\frac{1}{10}\right)

Rearranging the terms by moving the constants to the left and the fractions to the right, we get:   [1 \times 2 \times 3 \times \cdots \times 9] \times \left[\left(1-\frac{1}{2}\right) \times \left(1-\frac{1}{3}\right) \times \cdots \times \left(1-\frac{1}{10}\right)\right]

The right part of the expression is a telescoping product:   \left(\frac{1}{2}\right) \times \left(\frac{2}{3}\right) \times \left(\frac{3}{4}\right) \times \cdots \times \left(\frac{9}{10}\right) = \frac{1}{\not{2}} \times \frac{\not{2}}{\not{3}} \times \frac{\not{3}}{\not{4}} \times \cdots \times \frac{\not{9}}{10} = \frac{1}{10}

Therefore, the original expression simplifies to:   9! \times \frac{1}{10} = \frac{9!}{10} = \frac{362880}{10} = 36288

Final result: \boxed{36288}

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Problem 5 Easy

What is the value of \frac{3}{4} \times \frac{8}{9} \times \frac{15}{16} \times \frac{24}{25} \times \frac{35}{36} \times \frac{48}{49}?

  • A.

    \frac{3}{4}

  • B.

    \frac{8}{25}

  • C.

    \frac{1}{2}

  • D.

    \frac{4}{7}

  • E.

    \frac{48}{49}

Answer:D

By observation, we notice that all denominators are perfect squares.

We can attempt the following transformation: \frac{3}{4} \times \frac{8}{9} \times \frac{15}{16} \times \frac{24}{25} \times \frac{35}{36} \times \frac{48}{49}=\frac{1 \times 3}{2 \times 2} \times \frac{2 \times 4}{3 \times 3} \times \frac{3 \times 5}{4 \times 4} \times \frac{4 \times 6}{5 \times 5} \times \frac{5 \times 7}{6 \times 6} \times \frac{6 \times 8}{7 \times 7} = \frac{1}{2} \times \frac{3}{2} \times \frac{2}{3} \times \frac{4}{3} \times \frac{3}{4} \times \frac{5}{4} \times \frac{4}{5} \times \frac{6}{5} \times \frac{5}{6} \times \frac{7}{6} \times \frac{6}{7} \times \frac{8}{7}

Now we group terms to make the cancellation explicit: = \frac{1}{2} \times \left(\frac{3}{2} \times \frac{2}{3}\right) \times \left(\frac{4}{3} \times \frac{3}{4}\right) \times \left(\frac{5}{4} \times \frac{4}{5}\right) \times \left(\frac{6}{5} \times \frac{5}{6}\right) \times \left(\frac{7}{6} \times \frac{6}{7}\right) \times \frac{8}{7}

Each pair in parentheses cancels out completely: = \frac{1}{2} \times 1 \times 1 \times 1 \times 1 \times 1 \times \frac{8}{7}

After all cancellations, we are left with: = \frac{1}{2} \times \frac{8}{7} = \frac{4}{7}

Final result: \boxed{\frac{4}{7}}

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Problem 6 Easy

What is the value of \frac{2^{2}}{3^{2}-1} \times \frac{4^{2}}{5^{2}-1} \times \frac{6^{2}}{7^{2}-1} \times \cdots \times \frac{20^{2}}{21^{2}-1}?

  • A.

    \frac{1}{8}

  • B.

    \frac{1}{11}

  • C.

    \frac{1}{20}

  • D.

    \frac{1}{2}

  • E.

    \frac{1}{441}

Answer:B

By observing the expression, we notice that the denominators can be factored using the difference of squares formula (a^2 - b^2) = (a-b)(a+b).

Let's rewrite the expression accordingly:   \frac{2^{2}}{3^{2}-1} \times \frac{4^{2}}{5^{2}-1} \times \frac{6^{2}}{7^{2}-1} \times \cdots \times \frac{20^{2}}{21^{2}-1} = \frac{2^{2}}{(3-1)(3+1)} \times \frac{4^{2}}{(5-1)(5+1)} \times \frac{6^{2}}{(7-1)(7+1)} \times \cdots \times \frac{20^{2}}{(21-1)(21+1)} = \frac{2 \times 2}{2 \times 4} \times \frac{4 \times 4}{4 \times 6} \times \frac{6 \times 6}{6 \times 8} \times \cdots \times \frac{20 \times 20}{20 \times 22}

Now we can see a clear cancellation pattern emerging : = \frac{\not{2} \times 2}{\not{2} \times 4} \times \frac{\not{4} \times 4}{\not{4} \times 6} \times \frac{\not{6} \times 6}{\not{6} \times 8} \times \cdots \times \frac{\not{20} \times 20}{\not{20} \times 22}

After complete cancellation of common factors, we are left with:   = \frac{2}{4} \times \frac{4}{6} \times \frac{6}{8} \times \cdots \times \frac{20}{22}

Observing this telescoping product, we see that each numerator cancels with the denominator of the next fraction:   = \frac{2}{\not{4}} \times \frac{\not{4}}{\not{6}} \times \frac{\not{6}}{\not{8}} \times \cdots \times \frac{\not{20}}{22}

After all cancellations, only the first numerator and the last denominator remain:   = \frac{2}{22} = \frac{1}{11}

Final result: \boxed{\frac{1}{11}}

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Problem 7 Medium

What is the value of \left(1+\frac{2}{3}\right)\times\left(1+\frac{2}{4}\right)\times\left(1+\frac{2}{5}\right) \times \cdots \times\left(1+\frac{2}{22}\right)?

  • A.

    46

  • B.

    24

  • C.

    22

  • D.

    \frac{12}{11}

  • E.

    \frac{5}{3}

Answer:A

Let's evaluate the expression by first converting each term to an improper fraction: \left(1+\frac{2}{3}\right) \times \left(1+\frac{2}{4}\right) \times \left(1+\frac{2}{5}\right) \times \cdots \times \left(1+\frac{2}{22}\right) = \frac{5}{3} \times \frac{6}{4} \times \frac{7}{5} \times \cdots \times \frac{24}{22}

Now we can write this product as a single fraction: = \frac{5 \times 6 \times 7 \times \cdots \times 22 \times 23 \times 24}{3 \times 4 \times 5 \times \cdots \times 22}

We observe a clear cancellation pattern: = \frac{\not{5} \times \not{6} \times \not{7} \times \cdots \times \not{22} \times 23 \times 24}{3 \times 4 \times \not{5} \times \cdots \times \not{22}}

After complete cancellation of common factors in numerator and denominator, we are left with: = \frac{23 \times 24}{3 \times 4}

Now we can simplify the remaining expression: = 23 \times \frac{24}{12} = 23 \times 2 = 46

Final result: \boxed{46}

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Problem 8 Medium

What is the value of \left(1+\frac{1}{2}\right) \times\left(1-\frac{1}{2}\right) \times\left(1+\frac{1}{3}\right) \times\left(1-\frac{1}{3}\right) \times\cdots \times\left(1+\frac{1}{10}\right) \times\left(1-\frac{1}{10}\right)?

  • A.

    \frac{11}{20}

  • B.

    \frac{1}{2}

  • C.

    \frac{3}{2}

  • D.

    \frac{9}{10}

  • E.

    \frac{21}{10}

Answer:A

We immediately notice that this problem combines two types of telescoping products.

Let's rewrite the original expression: \left(1+\frac{1}{2}\right) \times \left(1-\frac{1}{2}\right) \times \left(1+\frac{1}{3}\right) \times \left(1-\frac{1}{3}\right) \times \cdots \times \left(1+\frac{1}{10}\right) \times \left(1-\frac{1}{10}\right) = \left(1+\frac{1}{2}\right) \times \left(1+\frac{1}{3}\right) \times \cdots \times \left(1+\frac{1}{10}\right) \times \left(1-\frac{1}{2}\right) \times \left(1-\frac{1}{3}\right) \times \cdots \times \left(1-\frac{1}{10}\right)

Now we convert each term to fraction form: = \frac{3}{2} \times \frac{4}{3} \times \cdots \times \frac{11}{10} \times \frac{1}{2} \times \frac{2}{3} \times \cdots \times \frac{9}{10}

We observe two separate cancellation patterns - one in the increasing sequence and one in the decreasing sequence. For the increasing sequence: \frac{\not{3}}{2} \times \frac{\not{4}}{\not{3}} \times \cdots \times \frac{11}{\not{10}} For the decreasing sequence: \frac{1}{\not{2}} \times \frac{\not{2}}{\not{3}} \times \cdots \times \frac{\not{9}}{10}

Combining both cancellations, we are left with: = \frac{11}{2} \times \frac{1}{10}

Multiplying the remaining terms gives us the final result: = \frac{11}{20}

Final result: \boxed{\frac{11}{20}}

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