After 5 years from today the ages of three friends will be in an AP(arithmetic progression), and their sum would be 72. What is the age of the middle friend today?
Let the ages after 5 years be a - d, a and a + d. The sum is given to us. So (a - d) + a + (a + d) = 3a = 72. We get a = 72/3 = 24. So, the age of the middle friend today is a - 5 = 19 years.