I hope this post doesn't come too late.
You already know that a constant speed is defined by:
Let be the speeds with which Dionne, Basha and Sandra are running. The running distance is measured in laps (L).
You know that
The relative speed between two runners is calculated by the difference of their speeds and because you can extract from the text 2 linear equations:
. Multiply the first equation by 5 and add columnwise:
That means Sandra is running twice as fast as Dionne.
Sandra and Dionne are running in the same direction. The difference of their speeds is the relative speed between them. When Sandra overtakes Dionne she has covered 1 lap more than Dionne:
Divide both sides by L and simplify the value in the bracket:
. That means: Every 120 s = 2 minutes Sandra overtakes Dionne. If her speed is constant she'll meet Dionne 5 times in 10 minutes.