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.