(a)Before reducion\(R_1=11.25cm\)

The required radius is \(R_1=\sqrt{n \lambda f_1}=\sqrt{(1)(632.8nm)(200)}=0.1125cm\)

It must be reduced by a factor of 100

(b)\(R_N=\sqrt{N} R_1=\sqrt{20}(11.25cm)=50.31cm\)