A can do a work in 20 days and B in 30 days. If they work on it together for 4 days, then the fraction of the work that is left is :	Choose the correct option.
A) 1 / 3	B) 2 / 3	C) 4 / 3	D) 5 / 3	E) 7 / 3

Rationale: "ans is : b a ' s 1 day ' s work = 1 / 20 b ' s 1 day ' s work = 1 / 30 ( a + b ) ' s 1 day ' s work = ( 1 / 20 + 1 / 30 ) = 1 / 12 ( a + b ) ' s 4 day ' s work = ( 1 / 12 * 4 ) = 1 / 3 therefore , remaining work = ( 1 - 1 / 3 ) = 2 / 3"
Answer: B