Ifound a place that will give me a 20 percent discount if i spend over 50 dollars. my bill is 75 dollars. how much money will i save?

X of 20% = y
x - y = 75

x = total amount
y = discounted amount

x * 20/100 = y
x * 0.2 = y
replace y with 0.2x in other equation

x - y = 75
1x - 0.2x = 75
0.80x = 75
x = 75/.8
x = 93.75

Check:
93.75 * 20/100 = 18.75
93.75 - 18.75 = 75
x = 93.75
y = 18.75

Total amount without discount is $93.75

//BTW not sure about this, but kind of make sense