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# GMAT: Math 36

180(simpler)-184, pgs. 176-177. Created by Sal Khan.

## Want to join the conversation?

• sorry, but am confused about q# 184... could anyone pls help me understand it? ty...
(1 vote)
• Here is an easier way to solve this:
Let M = 10x + y (e.g. 15 = 1 x 10 + 5 x 1 right?)
So, N = 10y + x (e.g. 51 = 5 x 10 + 1 x 1)

So M + N = 10x + y + 10y + x = 11x + 11y = 11(x + y)

So, our answer MUST be a multiple of 11.

Now, the easiest way to check if a 3 digit number is a multiple of 11 is to check if the digit in the tens place (the middle one) is the sum of the one in the units place and the one in the hundreds place. e.g. given the three digit number ABC it will be a multiple of 11 if and only if B = A+C.

In option A this is not true. Whereas all other options are multiples of 11. So, our answer is A.