My girlfriend and I are trying to figure out how to divide costs in such a way that we both end up paying the same percentage of our income on each expense, but both of us are very math challenged and can't figure out how to do this. To clarify, let's say I make 2500 per month, and she makes 2000. Rent is 1000 (50% of her income, and 40% of mine). How do we split the cost of rent in such a way that it ends up being the same percentage of our monthly income? We are trying to come up with a formula that we can enter into excel and have it calculate the amount each of us has to pay for each bill. Any help would be greatly appreciated.