P, Q, and R can complete a task in 10, 15, and 20 days, respectively. P and Q start working on the task together, and after 3 days, R joins, and P leaves. If they work together for 3 more days, what fraction of the task is left to be completed?

Let the total work to be done is 300 units.

So, P does = 300/10 = 30 units per day

Q does = 300/15 = 20 units per day,

R does = 300/20 = 15 units per day.

P and Q start working on the take and worked for 3 days. So the work done in 3 days = (30 + 20)*3 = 150 units.

Now, R joins and P left. So, the work done by R and Q in the next 3 days = (20 + 15)*3 = 105 units

Remaining work = 300 - 150 - 105 = 45 units

Fraction of the work remaining = 45/300 = 3/20