A solid iron rod is a cylinder of radius 3.5 cm and length 20 cm. Iron weighs 8 g per ${\text{cm}}^3$. Find the mass of the rod in kilograms. Take π = 22/7.

Volume = $(22/7)\times 3.5^2\times 20=38.5\times 20=770{\text{cm}}^3$, so the mass = $770\times 8=6160$ g. Since 1 kg = 1000 g, that is 6.16 kg. (6160 is the mass in grams, which is what the question does not ask for; 61.6 and 0.616 come from dividing by 100 or by 10000 instead of 1000.)