Two friends P & Q take a project and start working on it together. P alone can finish the project in 20 days while Q takes 5 days more than P to finish it. After working together on the project for some days, P has to leave. Q now takes 10 days to finish the remaining project alone. After how many days of working together did P leave?

In a days P does 1/20 work; And in 1 day Q does 1/25 work
As seen above, Q works alone for 10 days.
In 1 days Q completes (1/25) × 10 = 2/5 work
Remaining work = 1 - (2/5) = 3/5 = Done by P and Q together
Total Work done = Total days x Work done by all in 1 day

Let P and Q work together for total K days.

∴ 3/5 = K × (1/20 + 1/25)
K = 20/3 = 6(2/3) days = Days when P and Q worked together
Thus P leaves after 6(2/3) days.