If A is thrice as fast as B and together can do a work in 21 days. In how many days A alone can do the work?	Choose the correct choice from the following answers:
A) 36	B) 42	C) 28	D) 54	E) 45

Rationale: "a ’ s one day ’ s work = 1 / x b ’ s one day ’ s work = 1 / 3 x a + b ’ s one day ’ s work = 1 / x + 1 / 3 x = 1 / 21 = 3 + 1 / 3 x = 4 / 3 x = 1 / 21 x = 21 * 4 / 3 = 28 answer : c"
Answer: C