This one is a little difficult unless you know how rate works. This is seen more frequently in secondary level maths.

Suppose you run twice as fast as your classmate. This means that you take half as much time as your classmate, or in other words, your classmate takes twice as much time as you. This makes sense after all, because you cover twice as far the distance as your classmate over the same period of time.

Since W has only completed 5/9 of the race when S has completed the race, it follows that W takes 9/5 times as long as S, and therefore, W's speed is 5/9 times as fast as S's speed. This follows that S's speed is 9/5 times as fast as W's speed.

Now we deal with units.

S has a speed of 9 units, while W has a speed of 5 units.

The extra 4 units correspond to a speed of 100m/min.

4 units --> 100m/min
1 unit --> 25 m/min
5 units --> 125 m/min

so W's speed is 125 m/min.

You can use a race distance of 2250 m to calculate the relevant numbers and check the validity of the answer.