If I give my brother 5 dollars, then we will have the same amount of money. If instead he gives me 12 dollars, then I'll have twice as much money as he will have. How much money does my brother currently have (in dollars)?
Let x be the amount of money your brother currently has.
If you give your brother $5, then you will both have x+5 dollars.
If your brother gives you $12, then you will have x+12 dollars and he will have x−12 dollars.
We are given that x+5=x−12 and x+12=2(x−12). Solving the first equation, we get x=7. Substituting x=7 into the second equation, we get 7+12=2(7−12), which is true.
Therefore, your brother currently has $7.
