A can do a piece of work in 20days, and B can do it in 15days. How long will they take if both work together?	Choices.
A) 7 days	B) 8.6 days	C) 13 days	D) 5.6 days	E) 10 days

Rationale: a + b can do the work in 20 * 15 / 20 + 15 = 20 * 15 / 35 = 8.6 days answer is b
Answer: B