Calculate Sell from Margin %

Hi, This is one for the mathmaticians out there.

Sell £100

Purchase Cost £50

Selling fee (i.e.Amazon) of 15% of £100 = £15

Margin = (100 - (50+15))/100 or ((sell-cost)/sell)

The question is, how do you calculate the sell price from the margin % when the sell price has a 15% discount?

Thanks

Mat