Hi,
I have specific budget and I give an arbitrary number to the price of a gram of Gold (pgG) I wish to buy.
I also know that Full Gold = pgG * 7.5; Half Gold= pgG * 3.5; Qaurter Gold = pgG * 1.7 .
For example, if I have 20 dollars (budget), and the pgG is 10 dollars (then FG=75, HG=35, QG=17 dollars), I can buy 0 FG, 0 HG, and 1 QG..and in the end I am left with 20-17=3 dollars in my pocket...Now, what I wish to find here is, what is the mathematical expression to find how much I am left...

Question

Hi,
I have specific budget and I give an arbitrary number to the price of a gram of Gold (pgG) I wish to buy. 
I also know that Full Gold = pgG * 7.5; Half Gold= pgG * 3.5; Qaurter Gold = pgG * 1.7 . 
For example, if I have 20 dollars (budget), and the pgG is 10 dollars (then FG=75, HG=35, QG=17 dollars), I can buy 0 FG, 0 HG, and 1 QG..and in the end I am left with 20-17=3 dollars in my pocket...Now, what I wish to find here is, what is the mathematical expression to find how much I am left...