A friend who runs a small delivery business got a factoring quote last month: "just a 3% fee on your invoices." He was ready to sign — his credit card charges more than that monthly. I asked him one question: 3% compared to what? His bank had offered a business credit line at 9% APR. The factoring company quoted 3%. Those numbers aren't in the same unit — one is charged once per invoice, the other is charged per year. So I did what developers do: I built a calculator to convert one int...