Market maths

Download a 3 minute MP3 on the unitary method [1.3 Mb ] for solving simple problems.

The ‘unitary method’ just means ‘find the cost of a unit’. Typical problems include

  1. 5 lemons cost 95 pence, how much will 8 lemons cost?
  2. 2 lb of sprouts cost 80p, how much will 5 lb cost?

These problems are simple and students can solve them quickly. 4 minutes plus a worksheet is about as much lesson time as I would spend on this topic. Full understanding provides a basis or ‘hook’ for percentages, ratios and even fractions.

The podcast has some sound recorded in Birmingham Market mixed in at the beginning and end just to lighten things up – I just walked up and down the stalls with an external mic on the digital voice recorder, and I was surprised how good the resulting track was. I recorded the speech outdoors; not such a good idea as the microphone picks up the always present traffic rumble from a main road, although there is definitely no echo.

The script

Birmingham food market sells fruit and veg of all kinds, and each stall holder has their own way of pricing their wares. You need to use your mental arithmetic skills to get the best deal.

In GCSE Maths, we have problems like this; three kilos of peaches cost two pounds forty pence. How much will five kilos cost?

To answer this kind of question you always start by finding the cost per kilo. In this example, you divide two pounds forty pence by three to get eighty pence per kilo for the peaches. Once you know the cost per kilo, you multiply by the number of kilos you want to buy, in this case five kilos. I get eighty pence multiplied by five to be four pounds.

To summarise: divide the cost by the weight to find the cost per unit. Then multiply by the weight you want to buy.

Try this example: Two pounds of tomatoes cost seventy pence. How much will three pounds cost? [ a few seconds ]. I got one pound and five pence. I divided seventy by two to get thirty five pence per pound and then I multiplied the thirty five pence by three to get one pound five pence. I multiplied the thirty five pence by three mentally; three thirties are ninety pence and three five pences make fifteen and then I just added.

Now you try these – answers are at the end.

Question one. Seven oranges cost eighty four pence. How much will ten oranges cost?

Question two. Three kilos of potatoes cost one pound twenty pence. How much will seven kilos cost?

Question three. Twelve eggs cost one pound eighty pence. How much will eight eggs cost?

Question four. Two pounds of sprouts cost ninety six pence. How much will five pounds cost?

Answers: Question one, one pound twenty. Question 2, two pounds eighty. Question 3, one pound twenty. Question four, Two pounds forty pence.

This method is called the unitary method as it is based on finding the cost per unit. Look up unitary method in a textbook and practice some more examples. Now back to Birmingham Market.

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