Tom invests $10,000 in a savings account that offers 3.5 percent interest, compounded continuously.
In 10 years Tom will have earned $(blank) in interest, and in (blank) years the investment will double. (Use the rule of 70 where required.)

Respuesta :

Compound  interest  formula

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

 

Where


A= Future value
P = the Principal (the initial amount of money) 
r = annual interest rate

t = time

n= number of times compounded in one t






Remark
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r is generally a percentage like 3%, 7% etc and are applied in the formula as 0.03, 0.07..., 

the interest is compounded generally annually (n=1), quarterly (n=4), monthly (n=12), etc...

t is in years,

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Compounding continuously means that n tends (or goes) to infinity.

As n goes to infinity, the expression 
[tex]P(1+ \frac{r}{n} )^{nt}[/tex] is:

[tex]\displaystyle{ \lim_{n\to\infty}P(1+ \frac{r}{n} )^{nt}=Pe^{rt}[/tex]

where e = 2.718.... (I assume this is rule 70)
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In short, when n is infinite, that is when we compound continuously, the formula is :

[tex]A=Pe^{rt}[/tex], 

for P=$10,000, r=3.5/100=0.035, and t=10 years, we have:

[tex]A=Pe^{rt}=\displaystyle{ 10,000\cdot e^{0.035\cdot10}=14,190.7[/tex]

dollars.

To perform the last equation we make use of a scientific calculator.



Answer: $14,190.7

First we need to find the future value in order to find the interest earned

The formula of the future value of compound continuously is
A=p e^rt
A future value?
P present value 10000
E constant
R interest rate 0.035
T time 10 years
A=10,000×e^(0.035×10)
A=14,190.68

Now calculate the interest earned by subtracting the present value from the future value
Interest earned=A-p
Interest earned=14,190.68−10,000
Interest earned=4,190.68

The rule of 70 states that if you divide 70 by interest rate, you will get how many years the investment will double
70/rate=time
70÷3.5=20 years

So Tom will have earned $(4190.68) in interest, and in (20) years the investment will double.

Hope it helps!
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