Solution:

Given inner diameter of the pipe = 6 cm

So inner radius, r = 6/2 = 3 cm

Outer diameter = 10 cm

Outer radius, R = 10/2 = 5 cm

Let h be the height of the pipe.

Volume of pipe = (R²-r²)h

= ×(5²-3²)×h

= h(25-9)

= 16h cm³

Let r be the radius of solid cylinder.

Volume of solid cylinder = r²h

Since pipe is melted and changed into a cylinder, their volumes remains same.

r²h = 16h

r² = 16

r = 4 cm

Diameter = 2r = 2×4 = 8 cm

Hence the diameter of the cylinder is 8 cm.