P and Q can complete a task in 50 and 20 days respectively. In how many days can they complete 70% of the task if they work together?

Let total work to be done = 100 units

P can complete the work alone in 50 days

=> P's efficiency = $$\frac{100}{50} = 2$$ units/day

Q can complete the work alone in 20 days

=> Q's efficiency = $$\frac{100}{20} = 5$$ units/day

(P + Q)'s 1 day's work = 2 + 5 = 7 units/day

=> 70% of the work = $$\frac{70}{100} \times 100 = 70$$ units

$$\therefore$$ Time taken by them together to complete 70% of the work = $$\frac{70}{7} = 10$$ days

=> Ans - (D)