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Everyday maths 1
Everyday maths 1

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3.2 Comparing weights

By law, weights of goods for sale in the UK have to be in metric units: grams and kilograms.

Historically, however, most people used imperial measures of weight: in size order these are ounces, pounds and stones.

  • 16 ounces (oz) = 1 pound (lb)
  • 14 pounds = 1 stone (st)

You might still come across these weights sometimes.

An ounce is a bit less than 30 g. A pound is a bit less than half a kilogram.

Example: Two weight measurements

You have an old ladder with a label that says it can hold up to 20 stone. You weigh 80 kg. Can you safely use the ladder?

Hint: 1 st = 14 lb

Method

You need to work out roughly what 20 stone is in kilograms. First, you need to find out how much 20 stone is in pounds.

  • 20 × 14 = 280 lbs

One pound is equivalent to nearly half a kilogram, so next you need to divide the weight in pounds by 2:

  • 280 ÷ 2 = 140

The ladder will take about 140 kg – so you’re safe!

Now try the following activity. Remember to check your answers once you have completed the questions.

Activity 8: Converting weights

  1. An airline’s weight allowance for a piece of hand luggage is 5 kg. You have weighed your bag on some old bathroom scales and found that it is 7 lbs. Can you take it?
  2. You are using a recipe your grandmother wrote down. It calls for 4 oz sugar. You only have 150 g left. Do you have enough to make the recipe?

Answer

  1. A pound is a bit less than half a kilogram. To estimate what the weight allowance is in pounds, you need to multiply the amount in kilograms by 2:
    • 5 × 2 = 10 lbs

    Alternatively, you could estimate how much your bag weighs in kilograms by dividing the amount in pounds by 2:

    • 7 ÷ 2 = 3.5 kg

    Using either method, your bag doesn’t exceed the weight allowance.

  1. An ounce is a bit less than 30 g. To estimate how much sugar you need in ounces, you need to multiply the amount in ounces by 30:
    • 4 × 30 = 120 g

    Alternatively, you could estimate how much sugar you have in ounces by dividing the amount in grams by 30:

    • 150 ÷ 30 = 5 oz

    Using either method, you have enough sugar for the recipe.

Summary

In this section you have learned how to:

  • estimate and measure weight
  • use metric units of weight
  • know the relationship between grams and kilograms
  • convert from imperial to metric units of weight.