The question regards reverse-percentages. This requires you to realise what percentage of the original amount the sale price represents. If the original price is 100% and you have lost 20%, the sale price is 80% of the original.
To get from the original price to the sale price, you would multiply by 80%, or its respective decimal multiplier of 0.8. Therefore it follows that to go the other way (to go from the sale price to the original price), you need to divide by 0.8:
$636 / 0.8 = $795
In case you weren't sure how to find the decimal multiplier, all you have to do is divide the percentage by 100. So for a percentage of 156%, the decimal multiplier is 156 / 100 = 1.56
I hope this helps
