P, Q and R start from the same place X at (a) kmph, (a+b) kmph and (a+2b) kmph respectively.

Question

If Q starts p hours after P, how many hours after Q should R start, so that both Q and R overtake P at the same time? 

Answer is pa/a+2b 

Your faculty have posted the answer but I didn't get some part of the answer properly and in that answers how i come to know that we have to take T and t-p and t-p-x and one request is that can you explain question properly