Since Tom runs 1 round in 7 min, let distance of 1 round = 7 units
Tom runs ⬜⬜⬜⬜⬜⬜⬜ in 7 min
In 2 min, distance Tom ran = ⬜⬜
Remaining distance of the round to run = ⬜⬜⬜⬜⬜
But Kelvin, who started at the same point and time but in the opposite direction,met him there after 2 min.
So in that 2 min, Kelvin must have run ⬜⬜⬜⬜⬜, which is the same distance Tom will have to run to complete 1 round.

In 4 min, distance Tom ran = ⬜⬜⬜⬜
Remaining distance of the round to run = ⬜⬜⬜
But Bob who started at the same point and time but in the opposite direction,met him there after 4 min.
So in that 4 min, Bob must have run ⬜⬜⬜, which is the same distance Tom will have to run to complete 1 round.
So,
Tom ran ⬜⬜⬜⬜ in 4 min.
Bob ran ⬜⬜⬜ in 4 min.
Kelvin ran ⬜⬜⬜⬜⬜ in 2 min.
So in 4 min (which is twice of 2 min), Kelvin ran twice of that , which = ⬜⬜⬜⬜⬜ ⬜⬜⬜⬜⬜

Ratio of Tom speed : Bob : Kelvin =
⬜⬜⬜⬜ : ⬜⬜⬜ : ⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜
= 4 : 3 : 10

(Note : to find speed in terms of per minute, we would divide each distance by 4
Tom = 4 units ÷ 4 min = 1 unit/min = 4/4 units/min
Bob = 3 units ÷ 4 min = ¾ units/min
Kelvin = 10 units ÷ 4 min = 10/4 units/min
Realise that the ratio 4/4 : 3/4 : 10/4 still simplifies to 4 : 3 : 10.
So we don't actually have to find out the speed per min to get the ratio, since we are dividing all by the same number of minutes. Just need to compare their distances run in the same time. )