The ages of two friends are in the ratio 13:7. What is the age of the younger friend if the sum of their ages is 100 years?
Let the ages be 13r and 7r. The younger is 7r. We have been given their sum. So (13 + 7)r = 100. Solving, we get r = 5. The younger is 7 × 5 = 35 years.