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Consumer math

Compound interest

In the real world, simple interest is normally used for a single period of less than a year, such as 30 or 60 days, because it is calculated on the original amount only. If your interest accumulated annually, you'd get a very different sum.

Compound interest is interest that accumulates on the original amount and all money that accumulated during each period.

The formula for compound interest is:

A=p(1+r)t

where A represents the final amount, t represents the time in years, p is teh principal (starting amount), and r is the interest rate expressed as a decimal.

Keisha has $90 in a savings account. The interest rate is 10%, compounded annually. To the nearest cent, how much interest will she earn in 2 years? A= p(1 + r)t Ais the balance (final amount). p is the principal (starting amount). r is the interest rate expressed as a decimal. t is the time in years. The interest is the balance minus the principal. Write the rate as a decimal. 10% = 0.1 Calculate the balance.  A =  p ( 1 +  r )  t =$90
(
 1 + 0.1
)
2

=
$90  ( 1.1 ) 2 =$90
 ( 1.21 )

=  $108.90 Now use this to find the interest, which is the balance minus the principal.$108.90 – $90 =$18.90

The interest will be \$18.90.