The average of 30 numbers is 15. The average of the first 18 number is 10 and that of next 11 numbers is 20. The last number is :

[A]50

[B]52

[C]56

[D]60

**50**

Let the last number be x.

According to the question,

18 10 + 11 20 + x = 30 15

=> 180 + 220 + x = 450

=> 400 + x = 450

=> x = 450 – 400 = 50

Hence option [A] is correct answer.