You want to have $40,000 for a down payment on a house 4 years from now. if you can earn 5.6 percent, compounded annually on your savings, how much do you need to deposit today to reach your goal?

You need a compound interest formula solved for principal:
Principal = Total ÷ (1 + Rate) ^ years

Principal = 40,000,/ (1.056) ^ 4
Principal = 40,000 / 1.2435282985
Principal = 32,166.54
So, 32,166.54 deposited for 4 years at 5.6% interest (compounded annually) produces $40,000.

Source:
http://www.1728.org/compint2.htm

The solutions are (0, 4) and (0, -4)

Step-by-step explanation:

Solve this system using substitution:  

Take the negative of the first equation and add the result to the second one:

-x^2 + y^2 = 16 becomes x^2 - y^2 = -16

Then:

x^2    - y^2 = -16

-2x^2 + y^2 = 16

-x^2 = 0, so x must be 0.

By the first equation, if x = 0, -(0)^2 + y^2 = 16, or

y^2 = 16, or y = ±4

The solutions are (0, 4) and (0, -4)