A number consists of two digits whose sum is 3. If 9 is added to the number, the digits are interchanged. The number is:

Explanation

Let the tens digit be a and the units digit be b. Their sum gives a + b = 3. Adding 9 reverses the number: 10a + b + 9 = 10b + a, so b − a = 1. Solving the two equations gives a = 1 and b = 2, hence the number is 12.