An iron pipe of internal diameter is 2.8cm and uniform thikness 1mm is melted and a solid cylindical rod of same length is formed . Find the diameter of the rod ?
The volume of metal is π/4 (3.8^2 - 2.8^2)*length
so the rod has the same volume, making its diameter
π/4 d^2*length = π/4 (3.8^2 - 2.8^2)*length
or, more simply
d^2 = 3.8^2 - 2.8^2
Now just finish it off