A100 kg athlete runs, at constant speed of 10 m/s, around a circle of radius 20.0m. find the athlete's acceleration ?

In circular motion, the acceleration is centripetal, that is, directed
toward the center of the circle.

Centripetal acceleration is:  (speed²)/(radius) = (10 m/s)²/(20m) = 0.5 m/s² .

Also, it takes force, directed towards the center of the circle, to
keep him moving in a circle.

                   Force = (mass) x (acceleration)

  Centripetal force = (mass) x (speed²/radius)  = 

                                 (100 kg) x (0.5 m/s²)  =  50 newtons .

The runner has to continually provide that force, by digging his
sneakers into the gravel and pushing toward the center of the track. 
It helps if he leans slightly in toward the center as he runs.

The formula for acceleration is:
 
(final velocity-starting velocity)÷time

As newton's first law states a object in motion stays in motion