A can do a job in 30days and B in 40days. If they work on it together for 10 days, then the fraction of the work that is left is?  Choose the correct choice from the following answers:
A) 2 / 11	B) 3 / 13	C) 5 / 12	D) 7 / 10	E) 6 / 15

Rationale: "a ' s 1 day work = 1 / 30 b ' s 1 day work = 1 / 40 a + b 1 day work = 1 / 30 + 1 / 40 = 7 / 120 a + b 4 days work = 7 / 120 * 10 = 7 / 12 remaining work = 1 - 7 / 12 = 5 / 12 answer is c"
Answer: C