2021 AMC 10A Fall Problem 8

Below is the professionally curated solution for Problem 8 of the 2021 AMC 10A Fall, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2021 AMC 10A Fall solutions, or check the answer key.

All of the real AMC 8, AMC 10, AMC 12, and AIME problems in our complete solution collection are used with official legal permission of the Mathematical Association of America (MAA).

Concepts:digitsDiophantine Equationdivisibility

Difficulty rating: 1210

8.

A two-digit positive integer is said to be cuddly if it is equal to the sum of its nonzero tens digit and the square of its units digit. How many two-digit positive integers are cuddly?

00

11

22

33

44

Solution:

Let a b\underline{a} \ \underline{b} be a 22-digit cuddly number.

Then 10a+b=a+b2. 10a + b = a + b^2. Rearranging, we get 9a=b(b1). 9a = b(b - 1). This means that 99 divides either bb or b1b - 1 (33 cannot divide both bb and b1b - 1).

The only way this is possible is if b=9b = 9 (bb is one digit, so it can't be anything else). Checking, we get that 8989 is a cuddly number. This shows that there is only 11 two-digit cuddly number.

Thus, B is the correct answer.

Problem 8 in Other Years