Let R = external radius

And r = internal radius

Given:

R -r = 75mm = 0.075 m -----------(1)

pi(R^2 -r^2) = 0.164 sq.m. -------(2)

So,

pi[(R+r)(R-r)] = 0.164

pi[(R+r)(0.075)] = 0.164

R+r = 0.164 / 0.075pi

R +r = 0.696 m ---------------------(3)

(1) plus (3),

2R = 0.075 +0.696 = 0.771

R = 0.771 /2 = 0.3855 m -----------***

(3) minus (1),

2r = 0.696 -0.075 = 0.621

r = 0.621 /2 = 0.3105 m ------------***