1993 AMC 8 Problem 19

Attempt Problem 19 of the 1993 AMC 8 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 1993 AMC 8 solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

19.

What is the value of the following expression?
(1901+1902+1903+⋯+1993)−(101+102+103+⋯+193) \begin{gathered} \small (1901 + 1902 + 1903 + \cdots + 1993) \\ \small {}- (101 + 102 + 103 + \cdots + 193) \end{gathered}
167,400167{,}400
172,050172{,}050
181,071181{,}071
199,300199{,}300
362,142362{,}142
Answer: A
Concepts:pairing and groupingarithmetic sequence
Small hint:
Pair each term with the one below it: 1901−101,1901 - 101, 1902−102,1902 - 102, …\ldots
Big hint:
Each of the 9393 pairs has difference 18001800
Solution:
Each number in the first sum is exactly 18001800 more than the matching number in the second sum, and there are 9393 such pairs.
So the difference is 93×1800=167,400.93 \times 1800 = 167{,}400.
Thus, the correct answer is A .
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