Question

On a ground one racer runs a distance at 100m in 10 seconds what is used to measure time

Answer 1

The distance = 100 m
Time = 10 seconds
Speed = Distance/time
=> 100/10
=> 10 m/s

Now time would usually be measured by a clock and since presumably according to the question, the time was not measured on it.

So maybe we had the speed at which the racer runs, that is 10 m/s and we had the distance too.

Now, just by reversing the formula
Speed = distance/time => time = distance/speed

Now we get the answer 10 seconds.
So a formula or a clock (may read: watch) will be used to measure the time.