Let's call her friend's amount of money y. Since the friend always has 300 more than her, we can set up an equation that says y=x+300 As for figuring out how much money they have together, you could make an equation like so: x+7=m, m being money. I used guess and check to figure out if they wanted 1000 dollars between them, the girl would need $350, hence her friend would have 350+300, or 650. And 350+650 is 1000.