The average age of four boys A, B, C and D is 5 years and the average age of A, B, D and E is 6 years. C is 8 years old. The age of E is (in years) :

12

The average age of four boys A, B, C and D = 5 years

A + B + C + D = 20 years

 C = 8 years 

so,  A + B + 8 + D = 20 years

A + B+ D = 12 years

 The average age of A, B, D and  E = 6 years 

Now  A + B + D + E = 24 years

 A + B+ D = 12 years

E = 24 - 12 = 12 years.