Buying by Weight

Fruit, cheese and vegetables are often priced per kilogram, and the cost is the price per kilogram times the weight. Potatoes cost 1.60 dollars per kilogram and you buy 2.5 kilograms. Use c for cost: c = 2.5 × 1.6.

Work it out as whole numbers: 25 · 16 is 400. Two decimal places make 4.00, so c = 4: the potatoes cost 4 dollars. The estimate agrees, since 2 and a half kilograms at a bit over 1.50 should cost about 4.

Why multiply? Each kilogram costs 1.60, so 2 kilograms cost twice that and half a kilogram costs half of it. Multiplying by 2.5 does both at once. A price ending in a zero, like 1.60, goes on the board as 1.6.