The average of marks obtained by 120 boys was 36. If the average of marks of passed boys was 39 and that of failed boys was 15, the number of boys who passed the examination is?  Choose the correct answer:
A) 105
B) 110
C) 120
D) 130
E) 140

Rationale: "let the number of boys who passed = x . then , 39 x x + 15 x ( 120 - x ) = 120 x 36 24 x = 4320 - 1800 = > x = 2520 / 24 x = 105 . hence , the number of boys passed = 105 . answer : a"
Answer: A