Finding interest rate(s) for charge accounts (unpaid balance);determining mortgage insurance and mortgage payments (fixed rate) for 15, 20,25 years and 30 years - amount saved if mortgage is paid in 25 years instead of 30 years. To obtain bank loan - rule is 2 1/2 times yearly salary. To rent a house - renter's rule is rent should be no more than one week's wages. Financing a car - cost of car less down payment - amount left to borrow - determine monthly payments times length of loan (48-72 months) - then find deferred price of car.