A can do a work in 15 days and B in 20 days. If they work on it together for 5 days, then the fraction of the work that is left is :
Choose the correct choice from the following.
A) 1 / 3
B) 7 / 12
C) 5 / 12
D) 2 / 3
E) 1 / 2

Rationale: "ans is : c a ' s 1 day ' s work = 1 / 15 b ' s 1 day ' s work = 1 / 20 ( a + b ) ' s 1 day ' s work = ( 1 / 15 + 1 / 20 ) = 7 / 60 ( a + b ) ' s 5 day ' s work = ( 7 / 60 * 5 ) = 7 / 12 therefore , remaining work = ( 1 - 7 / 12 ) = 5 / 12 . answer : c"
Answer: C