out of 4 numbers , the average of first 3 is 20 and that of the last 3 is 15 . if the last number is 18 , the first number is :  Choose the most appropriate option.
A) 22 B) 33 C) 77 D) 99 E) 27

Rationale: explanation : let the numbers be a , b , c , d given , a + b + c = 60 , b + c + d = 45 now , d = 18 thus , b + c + 18 = 45 ⇒ b + c = 27 putting the value of b + c in a + b + c = 60 a + 27 = 60 ⇒ a = 33 answer : b
Answer: B