# Math Isu More Options: Make a Folding Card

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• Hey, Mrs. Leonard, I'm having some trouble with two of the math problems.
• Why don't you write it on the board?
• The amount you deposit each year in an annuity in the second year will be \$218.29 with 5% interest compounded annually after 18 years. The total balance in the account after 18 years is \$2813.24, Find the amount of interest made in the final year.
• Can you show me what you've done so far?
• First I found the amount you deposit each year. Then I took that amount and subtracted it from the amount in the last year and found the interest. But my answer doesn't match the answer given.
• t18= 2813.24      t2= 218.29          n = 16   I = 0.05218.29 = P(1.05)16            I = 2813.24 - 100P = 100                                I = 2713.24
• t18= 2813.24      t2= 218.29          n = 16   I = 0.05218.29 = P(1.05)16            I = 2813.24 - 100P = 100                                I = 2713.24
• Wait, t18 isn't the same as S18?
• I think you went wrong with your t18 value. I think you confused t18 for S18.
• S18= 2813.24      t2= 218.29          n = 16   I = 0.05218.29 = P(1.05)16     A = 100(1.05)1        I = 105 - 100P = 100                        A = 105                    I = 5
• The t18 that you used was actually the the total of the 18 years. Calculating the P value is correct but then you have to find out how much money was made in the last year. The n value is only 1 because there is only 1 year of gaining interest.
• S18= 2813.24      t2= 218.29          n = 16   I = 0.05218.29 = P(1.05)16     A = 100(1.05)1        I = 105 - 100P = 100                        A = 105                    I = 5
• After that you subtract your P value from the amount in the final year to find the interest made.
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