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?

Let the number be 10a + b where a is tens digit and b is units digit. Difference: (10a + b) - (10b + a) = 9a - 9b = 9(a - b) = 36. Therefore a - b = 4. The digit difference is always 4, regardless of which digits (as long as a > b to keep the difference positive). The key is that the difference is always a multiple of 9.