The digits of a two-digit number add up to 12, and the tens digit is to the units digit as 1 : 3. The number is:

Let the digits be k and 3k. Their sum is 4k = 12, so k = 3 and the digits are 3 and 9. As the tens digit is the smaller one, the number is 39. Check: 3 + 9 = 12 and 3 : 9 = 1 : 3. Writing 93 reverses the ratio to 3 : 1.