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Find the percent decrease. (Round to the nearest tenth of a percent.)
The population of North Dakota was about 672,000 in 2010. The population is projected to be about 630,000 in 2020.
6.3%
Find the percent decrease.
Last year, Sheila’s salary was $42,000. Because of furlough days, this year, her salary was $37,800.
10%
Do you know that banks pay you to keep your money? The money a customer puts in the bank is called the principal , P , and the money the bank pays the customer is called the interest . The interest is computed as a certain percent of the principal; called the rate of interest , r . We usually express rate of interest as a percent per year, and we calculate it by using the decimal equivalent of the percent. The variable t , (for time ) represents the number of years the money is in the account.
To find the interest we use the simple interest formula,
If an amount of money, P , called the principal, is invested for a period of t years at an annual interest rate r , the amount of interest, I , earned is
Interest earned according to this formula is called simple interest .
Interest may also be calculated another way, called compound interest. This type of interest will be covered in later math classes.
The formula we use to calculate simple interest is To use the formula, we substitute in the values the problem gives us for the variables, and then solve for the unknown variable. It may be helpful to organize the information in a chart.
Nathaly deposited $12,500 in her bank account where it will earn 4% interest. How much interest will Nathaly earn in 5 years?
Areli invested a principal of $950 in her bank account with interest rate 3%. How much interest did she earn in 5 years?
$142.50
Susana invested a principal of $36,000 in her bank account with interest rate 6.5%. How much interest did she earn in 3 years?
$7,020
There may be times when we know the amount of interest earned on a given principal over a certain length of time, but we don’t know the rate. To find the rate, we use the simple interest formula, substitute in the given values for the principal and time, and then solve for the rate.
Loren loaned his brother $3,000 to help him buy a car. In 4 years his brother paid him back the $3,000 plus $660 in interest. What was the rate of interest?
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