P and Q can finish a work in 24 and 12 days respectively. In how many days can they complete 50% of the work if they work together?

Let total work to be done = 24 units

P can complete the work alone in 24 days

=> P's efficiency = $$\frac{24}{24} = 1$$ unit/day

Q can complete the work alone in 12 days

=> Q's efficiency = $$\frac{24}{12} = 2$$ units/day

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

=> 50% of the work = $$\frac{50}{100} \times 24 = 12$$ units

$$\therefore$$ Time taken by them together to complete 50% of the work = $${12}{3} = 4$$ days

=> Ans - (C)