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Advanced Math / Nonlinear functions Difficulty: Medium

The function $f (t) = 40,000 {(2)}^{\frac{t}{790}}$ gives the number of bacteria in a population $t$ minutes after an initial observation. How much time, in minutes, does it take for the number of bacteria in the population to double?

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Explanation

Choice B is correct. It’s given that $t$ minutes after an initial observation, the number of bacteria in a population is $40,000 left parenthesis 2 right parenthesis Superscript StartFraction t Over 790 EndFraction$ . This expression consists of the initial number of bacteria, $40,000$ , multiplied by the expression $2 Superscript StartFraction t Over 790 EndFraction$ . The time, in minutes, it takes for the number of bacteria to double is the increase in the value of $t$ that causes the expression $2 Superscript StartFraction t Over 790 EndFraction$ to double. Since the base is $2$ , the expression $2 Superscript StartFraction t Over 790 EndFraction$ will double when the exponent increases by $1$ . Since the exponent of this expression is $StartFraction t Over 790 EndFraction$ , the exponent will increase by $1$ when $t$ increases by $790$ . Therefore, the time, in minutes, it takes for the number of bacteria in the population to double is $790$ .

Choice A is incorrect. This is the base of the exponent, not the time it takes for the number of bacteria in the population to double.

Choice C is incorrect. This is the number of minutes it takes for the population to double twice.

Choice D is incorrect. This is the number of bacteria that are initially observed, not the time it takes for the number of bacteria in the population to double.