1.3.1 Try some yourself
A local supermarket sells a popular breakfast cereal in a ‘Large Pack’ and ‘New Extra Large Pack’. They are both being sold at ‘knock down’ prices. The large pack contains 450 g of cereal priced at £1.85. The new extra large pack contains 625 g and is priced at £2.35. Which is the better bargain?
In order to compare the prices of the two cereal packs it is best to work out the price per gram for each.
The large packet will cost £1.85 for 450 g. This is or 0.411 p per gram.
The New extra large packet will cost £2.35 for 625 g. This is or 0.376 p per gram. Obviously the new extra large pack is the better bargain.
(But the extra large package is 37 cm tall (about 13˝). Unluckily it might not fit in the kitchen cupboard.)
Baking potatoes are priced at 75 p for a pack of three (very similar) potatoes or at £2.70 for a 5 kg bag. How heavy does each of the three potatoes in the pack have to be in order for the pack to be a better bargain than the 5 kg bag? Does your answer seem reasonable? (Try to imagine a potato of this size.)
5 kg costs £2.70. You want to know, at this rate, how much you get for 75 p. First work out how much you get for £1:
if £2.70 buys 5 kg then £1 buys kg.
So 75 p should buy 0.75 × 1.39 kg.
If three potatoes weigh 1.39 kg each, one weighs about 0.46 kg. So each potato in the pack would need to weigh around half a kilogram for the pack to be a better bargain.
Baking potatoes are big, but it's unlikely that a pack of three baking potatoes would weigh kg.