Gcse maths: more ratios

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Scaling up ratios

If you know the ratio between parts and the actual size of one part, you can scale the ratio up to find the other parts.

Put into practise 1 : Mortar is made from sand and cement in the ratio 7:2 if 21 buckets of sand are used, how much cement is needed?

You need to multiply by 3 to go from 7 to 21 in the left hand side - so do that to both sides.
7:3 X 3 21:6
So 6 buckets of cement are needed.

Practise 2: Mrs miggins owns tabby cats and ginger cats in the ratio 3:5 all her cats are either tabby or ginger, and she has 12 tabby cats. How many cats does Mrs Miggins have in total?

Multiply both sides by 4 to go from 3 to 12 on the LSH = 12:20
So Mrs Miggins has 12 tabby cats and 20 ginger cats. So in total she has 12 + 20 = 32 cats.

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Proportional division

Practise: Jess, Mo and Greg share £9100 in the ratio 2:4:7. How much does Mo get?

1) Add up the parts: the ratio 2:4:7 means there will be a total of 13 parts.

2) Divide to find one "Part": Just divide the total amount by the number of parts - £9100 / 13 = £700 (=1 part)

3) Multiply to find the amounts: We want to know Mo's share which is 4 parts
4 parts = 4 X £2800

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