My mother ordered me to go to the supermarket and buy food and some chocolates for me, she gave me a 20 USD bill, when I came back I brought what she asked for and 3.85 USD, my mother asked me how much I had spent and I did the following
Let x , the money I spent in the market
20=x+3.85
what I spent(x)= what my mother gave me -the money I brought
x=20USD-3.85
x=16.15 USD