we need some barriers such that total of length of all barriers should equal to the circumference of the circle.
so lets assume we need x barriers
then total length of all barriers = 2.5x meter
now radius of circular path = diameter / 2= 13/2 = 6.5 meter
circumference of path = 2× pi × r = 2 × 3.14 × 6.5
=40.82 meter
so now as per statement
2.5x = 40.82
x = 40.82/2.5 = 16.328 ≈ 17
so approximately 17 barriers are required