2019 AMC 10 A

Complete problem set with solutions and individual problem pages

Problem 5 Easy

What is the greatest number of consecutive integers whose sum is 45? (2019 AMC 10A Problem, Question#5)

  • A.

    9

  • B.

    25

  • C.

    45

  • D.

    90

  • E.

    120

Answer:D

We might at first think that the answer would be 9, because 1+2+3\cdots+n=45 when n=9. But note that the problem says that they can be integers, not necessarily positive. Observe also that every term in the sequence -44, -43, \cdots, 44, 45 cancels out except 45. Thus, the answer is, intuitively, 90 integers.

Though impractical, a proof of maximality can proceed as follows: Let the desired sequence of consecutive integers be a, a+1, \cdots, a+(N-1), where there are N terms, and we want to maximize N. Then the sum of the terms in this sequence is aN+\frac{(N-1)(N)}{2}=45.Rearranging and factoring, this reduces to N(2a+N-1)=90. Since N must divide 90, and we know that 90 is an attainable value of the sum, 90 must be the maximum.

To maximize the number of integers, we need to make the average of them as low as possible while still being positive. The average can be \frac{1}{2} if the middle two numbers are 0 and 1 , so the answer is \frac{45}{\frac{1}{2}}=90.