2009 AMC 10B Problem 7

Below is the professionally curated solution for Problem 7 of the 2009 AMC 10B, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2009 AMC 10B solutions, or check the answer key.

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Concepts:order of operationssystematic listing

Difficulty rating: 1050

7.

By inserting parentheses, it is possible to give the expression 2×3+4×52\times3+4\times5 several values. How many different values can be obtained?

22

33

44

55

66

Solution:

The three operations can be ordered in 3!=63!=6 ways, but performing the addition first or last leaves the two multiplications interchangeable, so at most four values arise.

Indeed (2×3)+(4×5)=26,(2×3+4)×5=50, (2\times3)+(4\times5)=26,\quad (2\times3+4)\times5=50, 2×(3+4×5)=46,2×(3+4)×5=70 2\times(3+4\times5)=46,\quad 2\times(3+4)\times5=70 are four distinct values.

Thus, the correct answer is C.

Problem 7 in Other Years