A proportion can be written as a formula. The formula will consist of two variables and a constant. The constant `k` is the multiplier to one of the variables:

`y = kx`

Write a conversion for a currency conversion from Euros to British Pounds. The conversion rate is 1.2 Euros for every British Pound.

Check that the conversion is correct: for £1, you would expect more Euros.

Answer: `p = 1.2e`

A formula is shown as `x = 0.25y`. Rewrite this direct proportion to make `y` the subject.

Multiply both sides of the proportion by 4.

Answer: `4x = y`

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