A, B and C can do a work in 90, 45 and 6 days respectively. If they work together, in how many days will they complete the work?  Pick one.
A) 5  B) 6  C) 7  D) 8  E) 11

Rationale: one days ' s work of a , b and c = 1 / 90 + 1 / 45 + 1 / 6 = ( 1 + 2 + 15 ) / 90 = 1 / 5 a , b and c together can do the work in 5 days . answer : a
Answer: A