What is the average speed of the swimmer between the times t=0 st=0\text{ s} t=0 s t, equals, 0, start text, space, s, end text and t=8 st=8\text{ s} t=8 s t, equals, 8, start text, space, s, end text ?

1 unit of lenght per second (it may be meters, but there is no enough information)

Explanation:

If t is time and s is position, we have that:

in t= 0s, s(t) = 0

in t= 8s, s(t) = 8

Then, we can see that the swimmer moved 8 units in the lapse of 8 seconds, and the average speed can be calculated as:

s = distance/time = 8units/8s = 1 unit per second.

So the average speed of the swimmer in that lapse of time is one unit per second, where unit depend on the problem, here may be metters.

i'm on my way to history not a mistory.