Question 138543
Since Paul paid at the time of purchase, he got the 20% discount.

He paid 400.

{{{400 = FullPrice - Discount}}}
{{{400 = Fullprice - 0.20*FullPrice}}}
{{{400 = 0.8* FullPrice}}}
{{{400/0.8 = FullPrice}}}
{{{500 = FullPrice}}}