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Elizabeth invested $970 in an account paying an interest rate of 6\tfrac{5}{8}6 8 5 ​ % compounded daily. Matthew invested $970 in an account paying an interest rate of 6\tfrac{3}{4}6 4 3 ​ % compounded continuously. After 8 years, how much more money would Matthew have in his account than Elizabeth, to the nearest dollar?

Sagot :

Using continuous compounding and compound interest, it is found that Matthew would have $17 more than Elizabeth in his account.

Compound interest:

[tex]A(t) = P\left(1 + \frac{r}{n}\right)^{nt}[/tex]

Continuous compounding:

[tex]A(t) = Pe^{rt}[/tex]

The parameters are:

  • A(t) is the amount of money after t years.  
  • P is the principal(the initial sum of money).  
  • r is the interest rate(as a decimal value).  
  • n is the number of times that interest is compounded per year.  
  • t is the time in years for which the money is invested or borrowed.

For both of them:

  • Investment of $970, hence [tex]P = 970[/tex]

  • Invested for 8 years, hence [tex]t = 8[/tex]

Elizabeth:

  • Compounded daily, hence [tex]n = 365[/tex].

  • Rate, as a percent, of [tex]6\frac{5}{8} = 6 + \frac{5}{8} = 6.625[/tex], hence [tex]r = 0.06625[/tex].

Then:

[tex]A(t) = P\left(1 + \frac{r}{n}\right)^{nt}[/tex]

[tex]A(8) = 970\left(1 + \frac{0.06625}{365}\right)^{365(8)}[/tex]

[tex]A(8) = 1648[/tex]

Matthew:

  • Rate, as a percent, of [tex]6\frac{3}{4} = 6 + \frac{3}{4} = 6.75[/tex], hence [tex]r = 0.0675[/tex].

Then:

[tex]A(t) = Pe^{rt}[/tex]

[tex]A(8) = 970e^{0.0675(8)}[/tex]

[tex]A(8) = 1665[/tex]

The difference is:

1665 - 1648 = 17

Hence, Matthew would have $17 more than Elizabeth in his account.

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