Explanation:
the arc is the part of the circle curve that is spanned by the central angle.
a full circle is 360° or 2pi radians.
and the full arc of a circle is the circumference
2pi×r
or
pi×2r = pi×diameter
a part arc is then
pi×diameter × angle/(2pi)
in our case
pi×diameter × (5pi/4)/(2pi) = 70.7 m
pi×diameter × (5pi)/(8pi) = 70.7 m
pi×diameter × 5/8 = 70.7 m
pi×diameter = 70.7×8/5 = 113.12 m (the full circumference)
diameter = 113.12/pi = 36.00721433... m ≈ 36 m