2016 AMC 8 Problem 8

Attempt Problem 8 of the 2016 AMC 8 below, then check your answer against the video solution and professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2016 AMC 8 solutions, or check the answer key.

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

8.

Find the value of the expression 100−98+96−94+92−90+⋯+8−6+4−2. \begin{gathered} 100 - 98 + 96 - 94 + 92 - 90 \\ {}+ \cdots + 8 - 6 + 4 - 2. \end{gathered}
20 20
40 40
50 50
80 80
100 100
Answer: C
Concepts:pairing and grouping
Small hint:
Group the expression into pairs of consecutive terms.
Big hint:
Count how many pairs of the form n−(n−2)n-(n-2) appear.
Video solution:
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Written solution:
We can group the sum as follows: (100−98)+(96−94)+⋯+(4−2). \begin{gathered} (100 - 98) + (96 - 94) \\ {}+ \cdots + (4 - 2). \end{gathered} Note that each pair evaluates to 22 and there are 2525 pairs. Therefore, the total sum is 2⋅25=50.2 \cdot 25 = 50.
Thus, C is the correct answer.
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