Compound Interest If you want to find t, the number of years, enter values for F, P and r.  Don't forget to change c, compounds per year. Formula from book where  i = r ÷ t  and n = t × c Example: You want to invest \$20,000 for 30 years  at 11 % interest compounded quarterly.  How much money will you have at the end of the 30 years?  (before taxes) We need to find i  and  n ... i = .11/4 = 0.0275  quarterly means 4 times a year (Hint: don't round until you are completely finished with your calculations)  P = present Value = 20,000  n = number of interest period = 30 × 4 = 120 We want to find F = Future value F = (1 + i)n × P = (1 + 0.0275)120 × 20,000 =  25.93102392 × 20,000 = 518,620.48 Try the calculator below.  Use P = present value = 20000 r = rate = .11, t =years = 30, c = compound periods = 4 Leave the F  blank. This is a Compound Interest calculator. Do not use commas or dollars signs in textboxes. F  = =  future value P = = present value r = = interest rate per year t = = number  of years c = 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 365 = compounds per year  monthly = 12, quarterly = 4 ANSWER: You must enter values in 3 out of the 4 textboxes. If you get NaN in the answer box , check your input values.  F should be more than P .  Interest rate should be between 0 and 1.  Years can be partial years i.e. 1 and a half years = 1.5

Annuities or Amortization
The present value P and the rent  R of a decreasing annuity of payments (rent) compounded at a rate i  per interest period.
Also called amortization when the payments are equal and at a regular time interval. (car loans and home mortgages )

James buys a house for \$90,000.  He puts \$10,000 down and then finances the rest at 9% interest compounded monthly for 25 years.

Find his monthly payments
This is a Present Value of an Annuity problem. We need to find the Rent R
 Formula from book where  i = r ÷ t  and n = t × c Note: n = 12(25) = 300 , i = .09/12 = .0075 and P = 90,000 - 10,000 = 80,000 The monthly payments are \$671.36

Find the total amounts he pays for the house.

You pay \$10,000 down and \$671.36 a month for 25 years
10,000 + 671.36 × 12 × 25 = 10,000 + 201,408 = 211,408
You pay \$211,408 for the house with interest.