A can do a work in 21 days and B alone can do it 28 days, they begin the work together, but A left after some days. B completed the remaining work in 21 days. After how many days did A leave ?	Pick one.
A) 10	B) 8	C) 3	D) 15	E) 16

Rationale: explanation : ( a + b ) 1 day work = { 1 / 21 + 1 / 28 } = 1 / 12 = 12 days is required for a & b b ’ s 1 day work = 1 / 28 b ’ s 21 days work = 21 / 28 1 - 21 / 28 = 7 / 28 - 1 / 4 1 / 4 * 12 = 3 days answer : option c
Answer: C