If A is thrice as fast as B and together can do a work in 15 days. In how many days A alone can do the work?
Select
A) 36
B) 20
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 / 15 = 3 + 1 / 3 x = 4 / 3 x = 1 / 15 x = 15 * 4 / 3 = 20 answer : b"
Answer: B