Think of it this way. You have 5 apples. You eat 2. How many do you have left? Three. How? Well, you took away two from five, or you subtracted them (5-2) and got 3. We will do the same here.
If it’s the decimal places that confuse you, fear not. You subtract them just the same as you would normal numbers. I added a picture and labelled which numbers you subtract from which to make it easier if that’s the problem. Hope this helped.
if every 1 gallon equal 4 quarts then 5 gallons is the same 20 quarts and since it said for every 1 quart equals 2 bucks and if thats true then 20 quarts equals