One simple model for a person running the 100 m dash is to assume the sprinter runs with constant acceleration until reaching top speed, then maintains that speed through the finish line.

Question

If a sprinter reaches his top speed of 10.8 m/s in 2.54 s, what will be his total time?

Answer 1

While accelerating, he runs 5.4*2.54 = 13.72 meters. (The average speed is half the maximum).
That leaves 86.28 m still to be run at 10.8 m/s. That will take 10.8/10.8 = 7.99 s.

The total time for the 100 meters is
7.99 + 2.54 = 10.53 s