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First, you can subtract 15% from 100% to get 85%, which represents the discounted purchase price % of the whole. In other words, your discounted purchase price is 85% of the original price.
Now, you can divide the discount price by the decimal .85 (percentage expressed as a decimal) In this case, you will have £85 divided by .85, and this will give you £100.
If you need another example, let me know!