Financial Magic Numbers
1. 20% Down Payment required to avoid private mortgage insurance (PMI).
PMI is short for private mortgage insurance. This is a type of insurance mortgage lenders require when homebuyers put down less than 20 percent of the home's purchase price.
Source: Bankrate (Basics of Private Mortgage Insurance)
2. The Rule of 72
The rule says that to find the number of years required to double your money at a given interest rate, you just divide the interest rate into 72. For example, if you want to know how long it will take to double your money at eight percent interest, divide 8 into 72 and get 9 years.
As you can see, the "rule" is remarkably accurate, as long as the interest rate is less than about twenty percent; at higher rates the error starts to become significant.
You can also run it backwards: if you want to double your money in six years, just divide 6 into 72 to find that it will require an interest rate of about 12 percent.
Source: MoneyChimp.com (includes calculator to test the rule)