10. What is the age of the youngest child?

The sum of the ages of 5 children born at the interval of 3 years each is 50 years. What is the age of the youngest child?

Answer: 
4 years.

Explanation:
Let the ages of the children be x, (x + 3), (x + 6), (x + 9) and (x + 12) years.
Then, x + (x + 3) + (x + 6) + (x + 9) + (x + 12) = 50
     =>5x = 20
     =>x = 4.
Therefore, Age of the youngest child = x = 4 years.