2021 AMC 10A Fall Problem 8

Attempt Problem 8 of the 2021 AMC 10A Fall 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 2021 AMC 10A Fall solutions, or check the answer key.

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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

Answer: B
Concepts:digitsDiophantine Equationdivisibility
Difficulty rating: 1210
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). Because consecutive integers cannot both be divisible by 3,3, one of bb and b1b-1 must contain both factors of 3.3.

For a digit b,b, this leaves b=0,1,b=0,1, or 9.9. The first two choices give a=0,a=0, which does not make a two-digit number. If b=9,b=9, then a=8.a=8. 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.

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