Everyday maths 1 (Wales)
Everyday maths 1 (Wales)

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3.4 Calculate using metric units of weight

You may need to carry out other calculations with weight. This may require you to convert between metric units, either before you carry out the calculation or at the end.

Example: Weight of ingredients

If you were to buy 750g of flour, 500g of sugar and 250g of butter, what is the total weight of these ingredients in kilograms?

Method

As all of the measurements are given in grams, you can add them together:

750 g + 500 g + 250 g = 1 500 g

The question asks for the final weight in kilograms. Knowing that 1 kilogram equals 1 000 grams, you now need to convert the amount in grams:

1 500 ÷ 1 000 = 1.5 kg

Example: A block of cheese

A deli has a 1.4 kg block of cheese. Three pieces, each weighing 250 g, are cut from it. How much does the remaining block of cheese weigh?

Method

The main block of cheese is 1.4 kg, so you need to convert this into grams:

1.4 kg × 1 000 = 1 400 g

The three pieces of cheese weigh:

250 g × 3 = 750 g

Taking this away from the original weight of the block of cheese gives you the answer:

1 400 g – 750 g = 650 g

Now try the following activity.

Activity 14: Carrying out calculations with weight

Calculate the answers to the following problems without using a calculator. You may wish to look back at Session 1 to remind you about how to carry out calculations with whole numbers and decimals [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] .

You may double-check your answers with a calculator if you need to. Remember to check your answers.

  1. Lily is making 3 kg of jam. The jam is made up of fruit and sugar. The weight of the fruit is 1 kg 800 g. How much sugar should she add to make the 3 kg of jam?
  2. Three parcels weigh 1.25 kg, 3.5 kg and 600g. What is the total weight of the parcels in kilograms?
  3. The hand luggage allowance is 7 kg for a particular airline. If you buy a cabin bag that weighs 3.1 kg, what is the maximum weight that you can pack?
  4. A puppy weighs 2.3 kg at seven weeks old. It puts on 800 g a week. How much will it approximately weigh at ten weeks old?

Answer

  1. You need to decide whether to convert everything into grams or kilograms first. Using Method 1, converting everything into grams, the total weight of the jam in grams will be:
    • 3 kg × 1 000 = 3 000 g
    The weight of the fruit is:
    • 1 kg × 1 000 = 1 000 g + 800 g = 1 800 g
    Now you can take the weight of the fruit away from the total weight needed:
    • 3 000 g – 1 800 g = 1 200 g
    If needed, you can convert to kilograms:
    • 1 200 g ÷ 1 000 = 1.2 kg
    Using Method 2, expressing the weight of the fruit in kilograms, the weight of the fruit is 1 kg 800 g, which is 1.8 kg. If you take the weight of the fruit away from the total weight of the jam needed, the answer is:
    • 3 kg – 1.8 kg = 1.2 kg
  2. You need to decide whether to convert everything into grams or kilograms first. Using Method 1, converting everything to grams first:
    • Parcel 1: 1.25 kg × 1 000 = 1 250 g

      Parcel 2: 3.5 kg × 1 000 = 3 500 g

      Parcel 3: 600g

    Add the weights of the parcels in grams:
    • 1 250 g + 3 500 g + 600 g = 5 350 g
    The question wants the answer in kilograms, you will need to convert:
    • 5 350 g ÷ 1 000 = 5.35 kg
    Using Method 2, converting everything to kilograms first:
    • Parcel 1: 1.25 kg

      Parcel 2: 3.5 kg

      Parcel 3: 600 g ÷ 1 000 = 0.6 kg

    Add the weights of the parcels in kilograms:
    • 1.25 kg + 3.5 kg + 0.6 kg = 5.35 kg
  3. If the maximum hand luggage is 7 kg and the case weighs 3.1 kg, then you can pack the following amount without going over the maximum limit:
    • 7 kg – 3.1 kg = 3.9 kg
    You may have worked this out in grams:
    • Maximum weight: 7 kg × 1 000 g = 7 000 g

      Weight of case: 3.1 kg × 1 000 = 3 100 g

      Amount of luggage: 7 000 g – 3 100 g = 3 900 g

  4. To work out the answer, convert the puppy’s weight at seven weeks into grams first:
    • 2.3 kg × 1 000 = 2 300 g
    The puppy puts on 800 g a week:
    • Week 8: 2 300 g + 800 g = 3 100 g

      Week 9: 3 100 g + 800 g = 3 900 g

      Week 10: 3 900 g + 800 g = 4 700 g

    You may want to express you answer in kilograms:
    • 4 700 g ÷ 1 000 = 4.7 kg

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 between grams and kilograms
  • calculate using metric weights.

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