If A is thrice as fast as B and together can do a work in 30 days. In how many days A alone can do the work? Select.
A) 36 B) 42 C) 28 D) 54 E) 40

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 / 30 = 3 + 1 / 3 x = 4 / 3 x = 1 / 30 x = 30 * 4 / 3 = 40 answer : e
Answer: E