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Of all eligible voters in your county, 
70% are currently registered to vote.
A recent poll predicts that the relationship between 
V, the percentage of all eligible voters who are registered to vote, and 
t, the number of years from now will be modeled by the following equation.

V=100-30*e^(-0.04 t)
In how many years will 
80% of all eligible voters in your county be registered to vote?
Give an exact answer expressed as a natural logarithm.
years

Of all eligible voters in your county, 70% 70 \% are currently registered to vote.\newlineA recent poll predicts that the relationship between V V , the percentage of all eligible voters who are registered to vote, and t t , the number of years from now will be modeled by the following equation.\newlineV=10030e0.04t V=100-30 \cdot e^{-0.04 t} \newlineIn how many years will 80% 80 \% of all eligible voters in your county be registered to vote?\newlineGive an exact answer expressed as a natural logarithm.\newlineyears

Full solution

Q. Of all eligible voters in your county, 70% 70 \% are currently registered to vote.\newlineA recent poll predicts that the relationship between V V , the percentage of all eligible voters who are registered to vote, and t t , the number of years from now will be modeled by the following equation.\newlineV=10030e0.04t V=100-30 \cdot e^{-0.04 t} \newlineIn how many years will 80% 80 \% of all eligible voters in your county be registered to vote?\newlineGive an exact answer expressed as a natural logarithm.\newlineyears
  1. Set up equation with VV: Set up the equation with VV equal to 8080. We are given the equation V=10030e(0.04t)V = 100 - 30\cdot e^{(-0.04t)} and we want to find the value of tt when VV is 8080. So, we set VV to 8080 and solve for tt. VV00
  2. Subtract to isolate exponential term: Subtract 100100 from both sides of the equation.\newline80100=10030e0.04t10080 - 100 = 100 - 30e^{-0.04t} - 100\newline20=30e0.04t-20 = -30e^{-0.04t}
  3. Divide sides by 30-30: Divide both sides by 30-30 to isolate the exponential term.\newline20/30=(30e(0.04t))/30-20 / -30 = (-30*e^{(-0.04t)}) / -30\newline23=e(0.04t)\frac{2}{3} = e^{(-0.04t)}
  4. Take natural logarithm: Take the natural logarithm of both sides to solve for tt.ln(23)=ln(e0.04t)\ln(\frac{2}{3}) = \ln(e^{-0.04t})
  5. Simplify right side: Use the property of logarithms that ln(ex)=x\ln(e^x) = x to simplify the right side of the equation.ln(23)=0.04t\ln(\frac{2}{3}) = -0.04t
  6. Divide sides by 0.04-0.04: Divide both sides by 0.04-0.04 to solve for tt.t=ln(23)0.04t = \frac{\ln(\frac{2}{3})}{-0.04}

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