My father was 31 years old when I was born. My mother's age was 18 when my sister, who is 6 years younger to me, was born. What is the difference between the ages of my parents?
If the age of my father is F, then my age is F - 31, so my younger sister's age is (F - 31) - 6, which is = F - 37. If my mother's age is M, then M = (my sister's age) + 18, i.e., M = (F - 37) + 18. We get F - M = 19 years.
This question can be solved directly also. The age of my father at the time of birth of my sister was 31 + 6 = 37. At the time my mother was 18 years. So the difference between their ages = 37 - 18 = 19 years.