We know that in a circle of radius r unit, if an arc of length l unit subtends an angle θ radian at the centre, then θ = l/r.
It is given that r = 75cm
(i) l = 10cm ∴ θ = 10/75 = 2/15 radian
(ii) l = 15cm ∴ θ = 15/75 = 1/5 radian
(iii) l = 21cm ∴ θ = 21/75 = 7/25 radian