Answer:
The maximum amount of red kryptonite present is 33.27 g after 4.93 hours.
Step-by-step explanation:
dy/dt = y(1/t - k)
separating the variables, we have
dy/y = (1/t - k)dt
dy/y = dt/t - kdt
integrating both sides, we have
∫dy/y = ∫dt/t - ∫kdt
㏑y = ㏑t - kt + C
㏑y - ㏑t = -kt + C
㏑(y/t) = -kt + C
taking exponents of both sides, we have
![\frac{y}{t} = e^{-kt + C} \\\frac{y}{t} = e^{-kt}e^{C} \\\frac{y}{t} = Ae^{-kt} (A = e^{C})\\y = Ate^{-kt}](https://tex.z-dn.net/?f=%5Cfrac%7By%7D%7Bt%7D%20%3D%20e%5E%7B-kt%20%2B%20C%7D%20%20%5C%5C%5Cfrac%7By%7D%7Bt%7D%20%3D%20e%5E%7B-kt%7De%5E%7BC%7D%20%5C%5C%5Cfrac%7By%7D%7Bt%7D%20%3D%20Ae%5E%7B-kt%7D%20%20%20%28A%20%3D%20e%5E%7BC%7D%29%5C%5Cy%20%3D%20Ate%5E%7B-kt%7D)
when t = 1 hour, y = 15 grams. So,
(1)
when t = 3 hours, y = 30 grams. So,
(2)
dividing (2) by (1), we have
![\frac{30}{15} = \frac{3Ae^{-3k}}{Ae^{-k}} \\2 = 3e^{-2k}\\\frac{2}{3} = e^{-2k}](https://tex.z-dn.net/?f=%5Cfrac%7B30%7D%7B15%7D%20%20%3D%20%5Cfrac%7B3Ae%5E%7B-3k%7D%7D%7BAe%5E%7B-k%7D%7D%20%5C%5C2%20%3D%203e%5E%7B-2k%7D%5C%5C%5Cfrac%7B2%7D%7B3%7D%20%3D%20e%5E%7B-2k%7D)
taking natural logarithm of both sides, we have
-2k = ㏑(2/3)
-2k = -0.4055
k = -0.4055/-2
k = 0.203
From (1)
![A = 15e^{k} \\A = 15e^{0.203} \\A = 15 X 1.225\\A = 18.36](https://tex.z-dn.net/?f=A%20%3D%2015e%5E%7Bk%7D%20%5C%5CA%20%3D%2015e%5E%7B0.203%7D%20%5C%5CA%20%3D%2015%20X%201.225%5C%5CA%20%3D%2018.36)
Substituting A and k into y, we have
![y = 18.36te^{-0.203t}](https://tex.z-dn.net/?f=y%20%3D%2018.36te%5E%7B-0.203t%7D)
The maximum value of y is obtained when dy/dt = 0
dy/dt = y(1/t - k) = 0
y(1/t - k) = 0
Since y ≠ 0, (1/t - k) = 0.
So, 1/t = k
t = 1/k
So, the maximum value of y is obtained when t = 1/k = 1/0.203 = 4.93 hours
![y = 18.36(1/0.203)e^{-0.203t}\\y = \frac{18.36}{0.203}e^{-0.203X1/0.203}\\y = 90.44e^{-1}\\y = 90.44 X 0.3679\\y = 33.27 g](https://tex.z-dn.net/?f=y%20%3D%2018.36%281%2F0.203%29e%5E%7B-0.203t%7D%5C%5Cy%20%3D%20%5Cfrac%7B18.36%7D%7B0.203%7De%5E%7B-0.203X1%2F0.203%7D%5C%5Cy%20%3D%2090.44e%5E%7B-1%7D%5C%5Cy%20%3D%2090.44%20X%200.3679%5C%5Cy%20%3D%2033.27%20g)
<u>So the maximum amount of red kryptonite present is 33.27 g after 4.93 hours.</u>