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 doublecheck your answers with a calculator if you need to. Remember to check your answers.
 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?
 Three parcels weigh 1.25 kg, 3.5 kg and 600g. What is the total weight of the parcels in kilograms?
 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?
 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
 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
 1 kg × 1 000 = 1 000 g + 800 g = 1 800 g
 3 000 g – 1 800 g = 1 200 g
 1 200 g ÷ 1 000 = 1.2 kg
 3 kg – 1.8 kg = 1.2 kg
 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
 1 250 g + 3 500 g + 600 g = 5 350 g
 5 350 g ÷ 1 000 = 5.35 kg

Parcel 1: 1.25 kg
Parcel 2: 3.5 kg
Parcel 3: 600 g ÷ 1 000 = 0.6 kg
 1.25 kg + 3.5 kg + 0.6 kg = 5.35 kg

 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

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
 To work out the answer, convert the puppy’s weight at seven weeks into grams first:
 2.3 kg × 1 000 = 2 300 g

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
 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.