Difference between revisions of "2021 Fall AMC 10A Problems/Problem 8"

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==See Also==
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{{AMC10 box|year=2021 Fall|ab=A|num-b=7|num-a=9}}
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Revision as of 16:36, 23 November 2021

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?

$\textbf{(A) }0\qquad\textbf{(B) }1\qquad\textbf{(C) }2\qquad\textbf{(D) }3\qquad\textbf{(E) }4$

Solution 1

Note that the number $\overline{xy} = 10x + y.$ By the problem statement, \[10x + y = x + y^2 \Rightarrow 9x = y^2 - y \Rightarrow 9x = y(y-1).\] From this we see that $y(y-1)$ must be divisible by $9.$ This only happens when $y=9.$ Then, $x=8.$ Thus, there is only $1$ cuddly number which is $89.$ Thus, the answer is $\boxed{\textbf{(B).}}$

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See Also

2021 Fall AMC 10A (ProblemsAnswer KeyResources)
Preceded by
Problem 7
Followed by
Problem 9
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
All AMC 10 Problems and Solutions

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