That would have been my guess! The difference, of course, is that Ken was 20 m behind Jim whenJimfinished the race, not when John finished the race. Since Ken finished behind Jim, he was swimming slower and so would have lost ground in the time between John and Jim finishing. When John finished, Ken must have been closer than 20 m behind Jim.

Let's see. Suppose John swims at speed v metres per second. Then he would have finished in 100/v seconds.

Since Jim was then 10 metres behind, he swam 90 meters in 100/v seconds and so was swimming at the rate of (90)(v/100)= (9/10)v. At that rate he will have completed the race in 100((10/9)v)= 1000/(9v) seconds.

Since Ken was 20 metres behind Jim, he swam 80 meters in 1000/9v seconds so he was swimming at the rate of (80)(9v/1000)= (72/100)v.

When John finished the race, in 100/v seconds, Ken had also been swimming for 100/v seconds and so had gone (100/v)(72/100)v= 72 metres. Ken was 100-72= 28 meters behind John!