So,
First of all, remember that:
Given a circle, its diameter is the double of its radius.
Graphically, this is:
So if we're given the diameter of a circle, to find its radius we just have to divide that value by 2.
For example:
a. If the diameter is 90 meters, then the radius will be equal to 45 meters.
b. If the diameter is 22.8 cm, then the radius will be equal to 11.4 cm.
c. If the diameter is 19 ft, then the radius will be equal to 9.5 ft.
d. If the diameter is 12.5 km, then the radius will be equal to 6.25 km.
e. If the diameter is 64 in, then the radius will be equal to 32 in.