A can do a piece of work in 4 days. B can do it in 5 days. With the assistance of C they completed the work in 2 days. Find in how many days can C alone do it?	Choose the correct choice from the following.
A) 87 days	B) 20 days	C) 16 days	D) 19 days	E) 36 days

Rationale: "c = 1 / 2 - 1 / 4 - 1 / 5 = 1 / 20 = > 20 days answer : b"
Answer: B