Askier starting from rest skis straight down a slope 50 meters long in 5.0 seconds. what is the magnitude of the acceleration.

His average speed all the way is

                   (50 meters) / (5 sec)  =  10 m/s .

But, if the acceleration is uniform all the way down, then

            Average speed  =  (1/2) (start speed + end speed)

Start speed  =  0.
So
             Average speed  =  (1/2) (0 + end speed)

                                     =  (1/2) of end speed .

                   10 m/s        = (1/2) of end speed

               End speed (at the bottom)  =  20 m/s .

Magnitude of acceleration = (change in speed) / (time for the change)

                                      =     (20 m/s)            /    (5 sec)

                                       =                4 m/s²  .