Let us take outer radius as R and inner radius as r.
Area of ring is A=π(R2−r2)
357.5=π(R2−r2)
357.5÷π=(R2−r2) --- (1)
Multiplying eq(1) by 10, we get
3575÷π=10×(R2−r2) ----(2)
Surface area of hollow cylinder =2πRh+2πrh+2π(R2−r2)
3575=2πRh+2πrh+2π(R2−r2)
3575÷π=2Rh+2rh+2(R2−r2) ----(3)
Equation eq (2) and eq (3), we get
10(R2−r2)=2Rh+2rh+2(R2−r2)
8(R2−r2)=2Rh+2rh
8(R2−r2)=28(R+r)
(R−r)=3.5
Thickness of the cylinder =3.5cm