CBSE Class 9 Answered

total surface area of hollow cylinder = curved suface area + 2(area of base rings) 

                                                                 = 2h(R+r) + 2(R² -r²).....................1

 also,

                                                                 = 2(R +r)(h + R  -r)...........................2

total surface area of hollow cylinder = 4620sq cm

so,

2h(R+r) + 2(R² -r²) = 4620

area of base rings = 115.5 sq cm

so,        2h(R+r) = 4620 - 2(115.5)

               2(R+r) = 4389/7                                          {here   h = 7cm} 

                               =627....................3

now from eq 2

               2(R +r)(h + R  -r) = 4620

                               (h + R  -r)=4620/627                     {from eq 3}

                                       R  -r = 7.36842 - 7

                                       R  -r = .36842 cm

so thickness of given cylinder = .36842 cm