A common use for proportionality is currency exchange rates. When you change your money at the Post Office or travel agent ‘commission free’, they will use a different exchange rate when they sell you foreign currency to when they buy it, thus making a profit.
Activity 10 Example problem: Japanese yen
Zoë is going on holiday from England to Japan.
Before leaving, she changes 1500 pounds sterling (GBP) into Japanese yen (JPY).
The exchange rate is 1 GBP = 177.415 JPY.
Calculate the amount of foreign currency Zoë takes on holiday.
This question is quite straightforward since you are given the unit rate for £1 in yen. All that is required here is to multiply the number of pounds by the rate, giving 266 122.50 yen.
If the rate was given in another form, for instance: 20 GBP = 3548.30 JPY, another step would be needed to calculate the unit rate.