The average age of  20 students in a class is 15 years. The average of 16 students is 12. What is the average age of remaining 4 students?

Solution:  Age of  20 students  $=15\times 20=300years$

Age of  16 students $=16\times 12=192years$

Age of  4 students  $=108years$

Average age of  4 students $=\frac{108}{4}=27years$