If A is thrice as fast as B and together can do a work in 12 days. In how many days A alone can do the work?  Options
A) 16 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 / 12 = 3 + 1 / 3 x = 4 / 3 x = 1 / 12 x = 12 * 4 / 3 = 16 answer : a"
Answer: A