How many 2-digit positive integers satisfy the above equation?
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3 A B ⟹ 3 A B 3 A B − 1 0 A ⟹ A = A B = 1 0 A + B = B = 3 B − 1 0 B
We note that for A to be a positive integer, B ≥ 4 . For B = 4 , ⟹ A = 2 and B = 5 , ⟹ A = 1 . For B ≥ 6 , 3 B − 1 0 B < 1 . Therefore, there are only 2 two-digit positive integers 15 and 24 that satisfy the equation above.