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Simple Interest

Definition (Interest Earned) The difference between the accumulated value of an investment after a period of time and the initial amount invested (principal) is called the interest earned during that period.

Example (Interest Earned)An initial deposit of 1,000 is made into an investment account. No withdrawals are made and no other principal is invested. At the end of three years, the account is worth 1,300. The interest earned during the three-year period is simple interest _gr_1.gif] simple interest _gr_2.gif] simple interest _gr_3.gif]

Definition (Effective Rate of Interest) The effective rate of interest is the amount of money that one unit of principal will earn during one investment period. Alternatively, it is the ratio of the amount of interest earned during the period to the amount of principal invested at the beginning of the period. simple interest _gr_4.gif]

Example (Effective Rate of Interest) (a) In Example 1.3, the effective rate of interest earned over the three-year period can be calculated as simple interest _gr_5.gif]
    (b) An initial deposit of 500 accumulates to 520 at the end of one year and 550 at the end of the second year. The effective rate of interest for the first year is simple interest _gr_6.gif] simple interest _gr_7.gif] simple interest _gr_8.gif]  The effective rate of interest for the second year is simple interest _gr_9.gif] simple interest _gr_10.gif] simple interest _gr_11.gif]The effective rate of interest for the two-year period is simple interest _gr_12.gif] simple interest _gr_13.gif] simple interest _gr_14.gif]
    Note that it was assumed that the interest earned in the first year was re-invested during the second year, so that the principal amount at the beginning of the second year was 520. In part (a) no such assumption is needed. Also, note that the effective rate of interest can be calculated for any time period; in part (b) it was calculated separately for the first year, second year, and the two-year period. Note that the simple interest _gr_15.gif] and simple interest _gr_16.gif] are annual rates while the simple interest _gr_17.gif]is a two-year rate. Topic 4, Equations of Value, will discuss methods of converting the simple interest _gr_18.gif]two-year rate to an annual rate. simple interest _gr_19.gif] simple interest _gr_20.gif]

Definition (Simple Interest) A pattern of interest accrual such that the amount of interest earned in each investment period is constant is called simple interest. If simple interest _gr_21.gif] is the initial investment and simple interest _gr_22.gif]is the constant interest amount, the accumulated value at the end of simple interest _gr_23.gif]investment periods is given by simple interest _gr_24.gif]

Example (Simple Interest) (a) If 1050 is invested for three years at 4%, find the accumulated value and total interest earned at the end of the period using simple interest.
    Under simple interest, the 4% simple interest rate determines the constant interest amount by simple interest _gr_25.gif] simple interest _gr_26.gif]The accumulated value after 3 years is simple interest _gr_27.gif] simple interest _gr_28.gif]The total interst earned in the three year period is simple interest _gr_29.gif]Note that simple interest _gr_30.gif]
    (b) If 100 is invested at 3% simple interest, find the accumulated value after 3 years and the effective rates of interest in the third and fourth year.
    The accumulated value after three years is given by the formula simple interest _gr_31.gif]where simple interest _gr_32.gif]and simple interest _gr_33.gif]Therefore,
    
simple interest _gr_34.gif] simple interest _gr_35.gif]

     The effective rate of interest in the third year is the amount of interest in the third year divided by the accumulated amount at the end of the second year.
     
simple interest _gr_36.gif] simple interest _gr_37.gif] simple interest _gr_38.gif]

     The effective rate of interest in the fourth year is the amount of interest in the fourth year divided by the accumulated amount at the end of the third year.
     
simple interest _gr_39.gif] simple interest _gr_40.gif]

    (c) If 1115 is invested to earn a constant 44.6 each year, how many years will it take for the account value to double?
    The situation described is simple interest, since the interest amount is constant. We want to solve for simple interest _gr_41.gif] given that the accumulated value is 2230.
    
simple interest _gr_42.gif]

simple interest _gr_43.gif]

    (d) If 2130 is invested to earn a constant rate of 4%, what is the accumulated value after 4 years? How many whole years will it take for the account value to become greater than 3000?
    The accumulated value after 4 years is given by the formula simple interest _gr_44.gif] where simple interest _gr_45.gif] and simple interest _gr_46.gif] Substituting, we get
    
simple interest _gr_47.gif]

    To find the number of years it will take for the account value to grow to over 3000, we must solve the equation
    
simple interest _gr_48.gif]

simple interest _gr_49.gif]

therefore, the number of whole years for the account value to grow to over 3000 is 11. simple interest _gr_50.gif]

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Cite this as:
Simple Interest
Published by Library of Math -- Online math organized by subject into topics.
Written by Smith, David A.
http://www.libraryofmath.com/simple-interest.html
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