P completes the work in 10 days.
P's rate =101 work per day
Q completes the work in 20 days.
Q's rate =201 work per day
When working together, their rates are added:
Combined rate =101+201
=202+201
=203 work per day
To find the total days needed:
Days =RateTotal Work
=2031
=1×320
=320 days
Therefore, P and Q working together can finish the work in 320 days.