P can give Q a start of 20 seconds in a kilometer race.
P can give R a start of 200 meters in the same kilometer race.
And Q can give R a start of 20 seconds in the same kilometer race.
At the start
1) P <-20s-> Q<-20s-> R Time domain
So even before P starts to run R has run for 40 seconds
2) P <-200m-> R Distance
So, R runs 200m in 40 sec.
By this it must be clear where the 40 sec came from.
If a problem states that A can give B a start of x sec. It means that A and B will reach the destination at the same time, when B runs for x sec before A starting.
Speed of A in this case should be greater than speed of B.
Same is the case with the distance.
How long does P take to run the kilometer?
Wish you all good luck