a)
Tom takes 7 min to run a round. So in 2 min , he ran 2/7 of the round.
Now he met Kelvin there. Since 1 - 2/7 = 5/7, Tom has 5/7 of the round left to complete. This also means Kelvin completed 5/7 of the round in 2 min but Kelvin did so in the reverse direction.
After 4 min from the start, Tom completed 4/7 of the round. 1 - 4/7 = 3/7 so he has 3/7 left to complete. He met Bob there so this means Bob completed 3/7 of the round in 4 min, but in the reverse direction.
So Bob's ran 3/7 of round in 4 min.
Tom ran 4/7 of round in 4 min.
Kelvin ran 5/7 of round in 2 min, so in 4 min, he ran twice of that.
He ran 5/7 x 2 = 10/7 or 1 3/7 of a round in 4 min.
Since time is taken is the same for all 3 in this comparison, we just compare the numerators of the fractions.
We can say that a round's distance is 7 units. In 4 mins Tom ran 4 units, Bob ran 3 units and Kelvin ran 10 units.
To find their speed we would divide by 4 or multiply by ¼, to get the distance per minute. But since the ratio would still be the same as we are dividing/multiplying the same way for all 3 fractions, we can just directly compare the units for the required ratio.
Ratio of Tom's speed to Kelvin's to Bob's = 4 : 10 : 3