Out of four numbers ,the average of first three is 20 and that of the last three is 15. If the last number is 18,the first number is : Choose one:
A) 22
B) 33
C) 77
D) 99
E) 27
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: 33
<result>B</result>
Out of four numbers ,the average of first three is 20 and that of the last three is 15. If the last number is 18,the first number is :