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.