2002 AMC 10A Problem 6

Cindy was asked by her teacher to subtract

3

from a certain number and then divide the result by

9.

Instead, she subtracted

9

and then divided the result by

3,

giving an answer of

43.

What would her answer have been had she worked the problem correctly?

Show solution

Solution

Let

x

be the number. Cindy computed

\dfrac{x-9}{3}=43,

x-9=129

and

x=138.

The correct computation is

\dfrac{138-3}{9}=\dfrac{135}{9}=15.

Thus, the correct answer is A.

Tagged: linear equation · work backwards