A can do a work in 15 days and B in 20 days. If they work on it together for 2 days, then the fraction of the work that is left is : Choose the correct option:  A) 23 / 20  B) 5  C) 23 / 30  D) 23 / 12  E) 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 2 day ' s work = ( 7 / 60 * 2 ) = 7 / 30 therefore , remaining work = ( 1 - 7 / 30 ) = 23 / 30 . ans : c"
Answer: C