A two digit number is three times the sum of its digits. Adding forty five reverses its digits. The number is

Let number be 10x plus y. Condition one says 10x plus y equals three times x plus y. That gives 7x equals 2y. So y equals 3.5x. Valid digits show x equals 2 and y equals 7. Check the second condition. 27 plus 45 equals 72 which is reversed. Hence the number is twenty seven.