P and Q can do a project in 60 and 30 days respectively. In how many days can they complete 90% of the project if they work together? 

Let total work to be done = 60 units

P can complete the work alone in 60 days

=> P's efficiency = $$\frac{60}{60} = 1$$ unit/day

Q can complete the work alone in 30 days

=> Q's efficiency = $$\frac{60}{30} = 2$$ units/day

(P + Q)'s 1 day's work = 1 + 2 = 3 units/day

=> 90% of the work = $$\frac{90}{100} \times 60 = 54$$ units

$$\therefore$$ Time taken by them together to complete 90% of the work = $${54}{3} = 18$$ days

=> Ans - (D)