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You put $8,000\$8,000 into a savings account that has an annual interest rate of 3%3\%, compounded annually. How many years will it take for your savings to increase to $12,000\$12,000? Use the formula A=P(1+rn)ntA = P\left(1 + \frac{r}{n}\right)^{nt}, where AA is the balance (final amount), PP is the principal (starting amount), rr is the interest rate expressed as a decimal, nn is the number of times per year that the interest is compounded, and tt is the time in years. Round your answer to the nearest hundredth.

Full solution

Q. You put $8,000\$8,000 into a savings account that has an annual interest rate of 3%3\%, compounded annually. How many years will it take for your savings to increase to $12,000\$12,000? Use the formula A=P(1+rn)ntA = P\left(1 + \frac{r}{n}\right)^{nt}, where AA is the balance (final amount), PP is the principal (starting amount), rr is the interest rate expressed as a decimal, nn is the number of times per year that the interest is compounded, and tt is the time in years. Round your answer to the nearest hundredth.
  1. Identify values: Identify the values of PP, rr, nn, and AA. P=8000P = 8000 r=0.03r = 0.03 n=1n = 1 A=12000A = 12000
  2. Use formula and solve: Use the formula A=P(1+rn)ntA = P\left(1 + \frac{r}{n}\right)^{nt} and solve for tt. 12000=8000(1+0.031)1t12000 = 8000\left(1 + \frac{0.03}{1}\right)^{1 \cdot t} 12000=8000(1.03)t12000 = 8000(1.03)^t
  3. Divide to isolate: Divide both sides by 80008000 to isolate (1.03)t(1.03)^t. 120008000=(1.03)t\frac{12000}{8000} = (1.03)^t 1.5=(1.03)t1.5 = (1.03)^t
  4. Take natural logarithm: Take the natural logarithm (ln\ln) of both sides to solve for tt. ln(1.5)=ln((1.03)t)\ln(1.5) = \ln((1.03)^t) ln(1.5)=tln(1.03)\ln(1.5) = t \cdot \ln(1.03)
  5. Divide to isolate: Divide both sides by ln(1.03)\ln(1.03) to isolate tt. t=ln(1.5)ln(1.03)t = \frac{\ln(1.5)}{\ln(1.03)} t0.4054650.029559t \approx \frac{0.405465}{0.029559} t13.72t \approx 13.72

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