Metal discs of diameter 1.75 cm and thickness 2 mm are melted together and cast into a solid block measuring 11 cm by 10 cm by 7 cm. How many discs were melted? Take π = 22/7.

Each disc is a cylinder of radius 0.875 cm and height 0.2 cm, so its volume = $(22/7)\times 0.875^2\times 0.2=2.40625\times 0.2=0.48125{\text{cm}}^3$. The block holds $11\times 10\times 7=770{\text{cm}}^3$, so the number of discs = 770/0.48125 = 1600. (Using 1.75 cm as the radius gives 400; leaving the thickness as 2 cm instead of 2 mm gives 160.)