Answer:
34
Explanation:
Since swapping the digits changes the value by 9, we know the difference between the digits is 9/9 = 1. Two factors of 12 that differ by 1 are 3 and 4. Adding 9 to 34 gives 43, so the number you're looking for is 34.
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About interchanging digits
Suppose a number with tens digit t and ones digit o has its digits interchanged. The old value is 10t+o, and the new value is 10o+t. If the latter is larger, then the difference is ...
(10o+t) -(10t+o) = (10-1)o -(10-1)t = 9(o-t)
That is, interchanging the digits changes the value by 9 times the difference between the digits. If the number gets larger, then the ones digit is the larger digit.