Answer:
Husband:
The husband will have 16.35 mg of caffeine in his body at 7 pm.
Woman:
The pregnant woman will have 51.33 mg of caffeine in her body at 7 pm.
Step-by-step explanation:
The amount of caffeine in the body can be modeled by the following equation:
![C(t) = C(0)e^{rt}](https://tex.z-dn.net/?f=C%28t%29%20%3D%20C%280%29e%5E%7Brt%7D)
In which C(t) is the amount of caffeine t hours after 8 am, C(0) is how much coffee they took and r is the rate the the amount of caffeine decreases in their bodies.
110 mg of caffeine at 8 am,
So ![C(0) = 110](https://tex.z-dn.net/?f=C%280%29%20%3D%20110)
Husband
Half life of 4 hours. So
![C(4) = 0.5C(0) = 0.5*110 = 55](https://tex.z-dn.net/?f=C%284%29%20%3D%200.5C%280%29%20%3D%200.5%2A110%20%3D%2055)
![C(t) = C(0)e^{rt}](https://tex.z-dn.net/?f=C%28t%29%20%3D%20C%280%29e%5E%7Brt%7D)
![55 = 110e^{4r}](https://tex.z-dn.net/?f=55%20%3D%20110e%5E%7B4r%7D)
![e^{4r} = 0.5](https://tex.z-dn.net/?f=e%5E%7B4r%7D%20%3D%200.5)
Applying ln to both sides
![\ln{e^{4r}} = \ln{0.5}](https://tex.z-dn.net/?f=%5Cln%7Be%5E%7B4r%7D%7D%20%3D%20%5Cln%7B0.5%7D)
![4r = \ln{0.5}](https://tex.z-dn.net/?f=4r%20%3D%20%5Cln%7B0.5%7D)
![r = \frac{\ln{0.5}}{4}](https://tex.z-dn.net/?f=r%20%3D%20%5Cfrac%7B%5Cln%7B0.5%7D%7D%7B4%7D)
![r = -0.1733](https://tex.z-dn.net/?f=r%20%3D%20-0.1733)
So for the husband
![C(t) = 110e^{-0.1733t}](https://tex.z-dn.net/?f=C%28t%29%20%3D%20110e%5E%7B-0.1733t%7D)
At 7 pm
7 pm is 11 hours after 8 am, so this is C(11)
![C(t) = 110e^{-0.1733t}](https://tex.z-dn.net/?f=C%28t%29%20%3D%20110e%5E%7B-0.1733t%7D)
![C(11) = 110e^{-0.1733*11} = 16.35](https://tex.z-dn.net/?f=C%2811%29%20%3D%20110e%5E%7B-0.1733%2A11%7D%20%3D%2016.35)
The husband will have 16.35 mg of caffeine in his body at 7 pm.
Pregnant woman
Half life of 10 hours. So
![C(10) = 0.5C(0) = 0.5*110 = 55](https://tex.z-dn.net/?f=C%2810%29%20%3D%200.5C%280%29%20%3D%200.5%2A110%20%3D%2055)
![C(t) = C(0)e^{rt}](https://tex.z-dn.net/?f=C%28t%29%20%3D%20C%280%29e%5E%7Brt%7D)
![55 = 110e^{10}](https://tex.z-dn.net/?f=55%20%3D%20110e%5E%7B10%7D)
![e^{10r} = 0.5](https://tex.z-dn.net/?f=e%5E%7B10r%7D%20%3D%200.5)
Applying ln to both sides
![\ln{e^{10r}} = \ln{0.5}](https://tex.z-dn.net/?f=%5Cln%7Be%5E%7B10r%7D%7D%20%3D%20%5Cln%7B0.5%7D)
![10r = \ln{0.5}](https://tex.z-dn.net/?f=10r%20%3D%20%5Cln%7B0.5%7D)
![r = \frac{\ln{0.5}}{10}](https://tex.z-dn.net/?f=r%20%3D%20%5Cfrac%7B%5Cln%7B0.5%7D%7D%7B10%7D)
![r = -0.0693](https://tex.z-dn.net/?f=r%20%3D%20-0.0693)
At 7 pm
7 pm is 11 hours after 8 am, so this is C(11)
![C(t) = 110e^{-0.0693t}](https://tex.z-dn.net/?f=C%28t%29%20%3D%20110e%5E%7B-0.0693t%7D)
![C(11) = 110e^{-0.0693*11} = 51.33](https://tex.z-dn.net/?f=C%2811%29%20%3D%20110e%5E%7B-0.0693%2A11%7D%20%3D%2051.33)
The pregnant woman will have 51.33 mg of caffeine in her body at 7 pm.