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?

Answer: 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.