We will call:
S: Runner Sophia.
T: Runner Tyler.
At the beginning the distance between both runners is 400m
At t = 1s Sophia runs 5m and Tyler 4m
At t = 2s Sophia runs 10m and Tyler 8m
and so on.
So, for example at t = 1s, the distance between both runner is:
D = 400m - 5m(the advantage of Sophia) + 4m(what Tyler recovers)
So, we can find a general equation for this matter. Knowing that the distance, velocity and time are related by this equation:
[tex]d=vt[/tex]
Then, the distance between both runners, at any time, is given by:
[tex]D = 400-5t+4t[/tex]
So we need to find the how long it will take Sophia to lap Tyler. This happens when D = 0:
[tex]0 = 400 - 5t+4t[/tex]
∴ [tex]400-t = 0[/tex]
∴ [tex]t = 400s[/tex]