The average age of an adult class is 40years. 15new students with an avg age of 32years join the class. Therefore decreasing the average by 4year. Find what was theoriginal strength of class? Choose the correct choice from the following choices.
A) 8 B) 12 C) 15 D) 17 E) 18

Rationale: "let original strength = y then , 40 y + 15 x 32 = ( y + 15 ) x 36 â ‡ ’ 40 y + 480 = 36 y + 540 â ‡ ’ 4 y = 60 â ˆ ´ y = 15 c"
Answer: C