Thrice the Product

3 × A × B = A B 3 \times A \times B = \overline{AB}

How many 2-digit positive integers satisfy the above equation?

1 4 3 2

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1 solution

3 A B = A B 3 A B = 10 A + B 3 A B 10 A = B A = B 3 B 10 \begin{aligned} 3AB & = \overline{AB} \\ \implies 3AB & = 10A+B \\ 3AB - 10A & = B \\ \implies A & = \frac B{3B-10} \end{aligned}

We note that for A A to be a positive integer, B 4 B \ge 4 . For B = 4 B=4 , A = 2 \implies A = 2 and B = 5 B=5 , A = 1 \implies A = 1 . For B 6 B \ge 6 , B 3 B 10 < 1 \dfrac B{3B-10} < 1 . Therefore, there are only 2 \boxed{2} two-digit positive integers 15 and 24 that satisfy the equation above.

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