Circumference = 2 × pi × radius
C = 15
15 = 2 × pi × r
15÷2 = pi × r
7.5 = pi × r
7.5÷pi = r
Radius equals to 7.5 divided by pi.
You can also use this formula for circumference --> C = d × pi where d is the diameter. Then you'll have to divide your answer by 2, to get the radius.
Hope i helped :)