The sale price of an item is four-fifths the original price of the item x

We have to multiply 4/5 by x to find out its new price

Picture it like X is now 20% off, that would be 1/5 as a percent, so it stands to reason that x multiplied by 4 and then divided by 5 is the new price. There are a few answers to this, such as turning the fraction into a decimal form of a percent and using it as a coefficient like so:
0.8x = Price

Or

(4/5)x = new price

Or

X - (1/5)X = new price