Average age of 7 students of a class is 28 years. Average age of first three students is 30 years. Age of fourth student is 4 years less than the age of fifth student. Ages of last two students is same and is 5 more than the average age of first three students. What is the average age of fourth and fifth student?

Total age of 7 students = 7 x 28 = 196 years
Total age of first 3 students = 3 x 30 = 90 years
5^th student age = x years
4^th student age = (x + 4) years
Age of 6^th and 7^th student = 30 + 5 = 35 years
Total age of 7 students = 196
⇒ 90 + (x + 4) + (x) + 35 + 35 = 196
⇒ 2x = 196 - 164
⇒ 2x = 32
∴ x = 16 years
Average of 4^th and 5^th student
$$\eqalign{ & = \frac{{\left( {x + 4} \right) + x}}{2} \cr & = \frac{{2 \times 16 + 4}}{2} \cr & = 18{\text{ years}} \cr} $$