In 75 seconds Sam will run 550 meters...
If Sam can run 200 meters in 20 seconds he can run 100 meters in 10 seconds and 50 meters in 5 seconds and he can run 400 meters in 60 seconds...
With that, in 75 seconds you would take the amount he can run in 60 seconds (400 meters) the amount he can run in 10 seconds (100 meters) and the amount he can run in 5 seconds (50 meters) and add them all up...
which gives you 550 meters for 75 seconds
