1994 AMC 12 Problem 14

Attempt Problem 14 of the 1994 AMC 12 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1994 AMC 12 solutions, or check the answer key.

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14.

Find the sum of the arithmetic series 20+2015+2025+⋯+40. 20+20\frac15+20\frac25+\cdots+40.
30003000
30303030
31503150
41004100
60006000
Answer: B
Concepts:arithmetic seriesmixed numbers
Small hint:
The common difference is 15\frac{1}{5}
Big hint:
Find the number of terms, then multiply by the average of the endpoints
Solution:
The number of terms is 40−2015+1=101. \frac{40-20}{\frac{1}{5}}+1=101. Their average is 20+402=30,\frac{20+40}{2}=30, so the sum is 101⋅30=3030.101\cdot30=3030. Thus the correct answer is B.
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