The difference between a two digit number and the one obtained by reversing its digits is 81. If one of the digits is 4, what is the other?
Let the numbers be a and b. ab = 10a + b, and ba = 10b + a. The difference is 10a + b - (10b + a) = 9(a - b). It is given that 9(a - b) = 81, ⇒ a - b = $81/9$ = 9, ⇒ the difference between the digits is 9. If one of the digits is 4, the other cannot be determined.