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The Browns wish toaccumulate at least $150,000 at the time of their last deposit in a college fund for their daughter by contributing an amount A into the account at the end of each year for eighteen years. What is the smallest annual payment A that will suffice if the college fund earns a level annual effective interest rate of 5%? If at the end of ten years, it is announced that the annual effective interest rate will drop to 4.5%, how much must the Browns increase their payments in order to reach their accumulation goal? Assume that the Browns wish to continue to make level payments except for a slightly reduced final payment.

Sagot :

Answer:The smallest initial annual payment ( A ) required at a 5% interest rate is approximately $5,330.88. If the interest rate drops to 4.5% after 10 years, the Browns need to increase their annual payments by approximately $3,304.28 to reach their goal of $150,000.

Step-by-step explanation: To solve this problem, we need to break it down into two parts:

1. Finding the initial annual payment (A) with a 5% interest rate for the entire 18 year.

2. Recalculating the required payment after the interest rate changes to 4.5% after 10 years.

Part 1: Initial Annual Payment with a 5% Interest Rate

The future value of an ordinary annuity can be calculated using the formula:

[ FV = A cdot frac{(1 + r)^n - 1}{r} ]

where:

- ( FV ) is the future value of the annuity,

- ( A ) is the annual payment,

- ( r ) is the annual effective interest rate,

- ( n ) is the number of payments.

Given:

- ( FV = 150,000 ) dollars,

- ( r = 0.05 ),

- ( n = 18 ).

Substituting these values into the formula, we get:

[ 150,000 = A cdot frac{(1 + 0.05)^{18} - 1}{0.05} ]

Let's solve for ( A ):

[ 150,000 = A cdot frac{(1.05)^{18} - 1}{0.05} ]

Calculating the value of ((1.05)^{18}):

[ (1.05)^{18} approx 2.4066 ]

So,

[ 150,000 = A cdot frac{2.4066 - 1}{0.05} ]

[ 150,000 = A cdot frac{1.4066}{0.05} ]

[ 150,000 = A cdot 28.132 ]

Finally,

[ A = frac{150,000}{28.132} approx 5,330.88 ]

Part 2: Adjusted Payments After Interest Rate Drops

After 10 years, the Browns have already accumulated some amount which we need to calculate. Then we will adjust the payment for the remaining 8 years at the new interest rate.

Step 1: Calculate the accumulated amount after 10 years at 5% interest rate

Using the future value formula for the first 10 years:

[ FV_{10} = A cdot frac{(1 + r)^{10} - 1}{r} ]

[ FV_{10} = 5,330.88 cdot frac{(1.05)^{10} - 1}{0.05} ]

Calculating ((1.05)^{10}):

[ (1.05)^{10} approx 1.6289 ]

So,[ FV_{10} = 5,330.88 cdot frac{1.6289 - 1}{0.05} ]

[ FV_{10} = 5,330.88 cdot frac{0.6289}{0.05} ]

[ FV_{10} = 5,330.88 cdot 12.578 approx 67,059.62 ]

Step 2: Calculate the new annual payment needed for the next 8 years at 4.5% interest rate

The remaining amount needed is:

[ 150,000 - 67,059.62 = 82,940.38 ]

Using the future value formula with the new interest rate ( r = 0.045 ) and ( n = 8 ):

[ 82,940.38 = A_{new} cdot frac{(1 + 0.045)^8 - 1}{0.045} ]

Calculating ((1.045)^8):

[ (1.045)^8 approx 1.4323 ]

So,

[ 82,940.38 = A_{new} cdot frac{1.4323 - 1}{0.045} ]

[ 82,940.38 = A_{new} cdot frac{0.4323}{0.045} ]

[ 82,940.38 = A_{new} cdot 9.6067 ]

Finally,

[ A_{new} = frac{82,940.38}{9.6067} approx 8,635.16 ]

Increase in Payments

The Browns must increase their annual payment from ( 5,330.88 ) to ( 8,635.16 ).

[ text{Increase in payment} = 8,635.16 - 5,330.88 approx 3,304.28]

Conclusion

The smallest initial annual payment ( A ) required at a 5% interest rate is approximately $5,330.88. If the interest rate drops to 4.5% after 10 years, the Browns need to increase their annual payments by approximately $3,304.28 to reach their goal of $150,000.

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