2014 AMC 8 Problem 22

Attempt Problem 22 of the 2014 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 2014 AMC 8 solutions, or check the answer key.

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

22.

A 22-digit number is such that the product of the digits plus the sum of the digits is equal to the number. What is the units digit of the number?
1 1
3 3
5 5
7 7
9 9
Answer: E
Concepts:digitsalgebraic manipulation
Small hint:
Let the tens digit be aa and the units digit be bb.
Big hint:
Translate the condition into ab+a+b=10a+bab+a+b=10a+b.
Solution:
Let the number be 10a+b10a+b, where aa is the tens digit and bb is the units digit. The condition gives ab+a+b=10a+b.ab+a+b=10a+b.
Canceling bb from both sides gives ab+a=10aab+a=10a, so ab=9aab=9a. Since a≠0a\neq0, we get b=9b=9.
Thus, E is the correct answer.
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