AMC 8 L1: Calculation Tricks

Complete problem set with solutions and individual problem pages

Problem 2 Easy

On her first day of work, Janabel sold one widget. On day two, she sold three widgets. On day three, she sold five widgets, and on each succeeding day, she sold two more widgets than she had sold on the previous day. How many widgets in total had Janabel sold after working 20 days? (2015 AMC 8 Problem, Question #9)

  • A.

    39

  • B.

    40

  • C.

    210

  • D.

    400

  • E.

    401

Answer:D

Solution 1: 1+3+5+\cdots +\left[ \left( 20-1 \right)\times 2+1 \right] =1+3+5+\cdots +39 =\left( 1+39 \right)\times 20\div 2 =400.

Solution 2: The sum is just the sum of the first 20 old integers, which is {{20}^{2}}=\text{D} 400.

Related Problems
Problem 1 AMC 8 L1: Calculation Tricks
Problem 3 AMC 8 L1: Calculation Tricks
Problem 4 AMC 8 L1: Calculation Tricks
Problem 5 AMC 8 L1: Calculation Tricks
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