Solution:

Given radius of the sphere, r = 6 cm

Volume of the sphere = (4/3)r³

= (4/3)×6³

= 288 cm³

Let r be the internal radius of the hollow cylinder.

External radius of the hollow cylinder, R = 4 cm

Height of hollow cylinder, h = 72 cm

Volume of hollow cylinder = (R²-r²)h

Since sphere is melted and changed into a hollow cylinder, their volumes remain same.

(R²-r²)h = 288

(4²-r²)×72 = 288

(4²-r²) = 288/72

(4²-r²) = 4

16-r² = 4

r² = 16-4

r² = 12

r = 2√3 cm

So thickness = R-r = 4-2√3

= 4- 3.464

= 0.536 cm

= 0.54 cm (approx)

Hence the thickness of the cylinder is 0.54 cm.