The difference between a two-digit number and the number obtained by interchanging the positions of its digits is 36. What is the difference between the two digits of that number?

A.    3   
B.    4
C.    9   
D.    Cannot be determined
E.    None of these

Ans: B

Explanation:

Let the ten's digit be x and unit's digit be y.
Then, (10x + y) - (10y + x) = 36
9(x - y) = 36
x - y = 4.