A solid metal cylinder of radius 6 cm and height 45 cm is melted and drawn into a wire of uniform diameter 4 mm. What is the length of the wire, in metres?

The cylinder holds π $x36\times 45=1620$π cubic cm. The wire's radius is 4 mm / 2 = 0.2 cm, so its cross-section is π $x$ 0.04 = 0.04π sq cm. Length = 1620π/0.04π = 40500 cm = 405 m. The trap 40500 is that answer left in centimetres; 101.25 takes 4 mm as the RADIUS rather than the diameter; 4.05 reads the 4 mm as 4 cm.