P and Q can do a project in 6 and 12 days respectively. In how many days can they complete 25% of the project if they work together?

Let total work to be done = 12 units

P can complete the work alone in 6 days

=> P's efficiency = $$\frac{12}{6} = 2$$ units/day

Q can complete the work alone in 12 days

=> Q's efficiency = $$\frac{12}{12} = 1$$ unit/day

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

$$\therefore$$ Time taken by them together to complete 25% of the work = $$\frac{25}{100} \times \frac{12}{3}$$

= $$\frac{1}{4} \times 4 = 1$$ day