Answer is 20.0 km per hour.
Explanation;
S = d/t
Where, S is the speed, d is the distance and t is the time taken.
Here,
d = 50.0 km
t = 2.5 hours
By applying the formula,
S = 50.0 km / 2.5 hours
= 20.0 km per hour
Hence, the average speed of the cyclist is 20.0 km per hour.