Step-by-step explanation
Part A
The formula for an exponential decay model is given as:

Where C is the initial amount and r is the decay rate and t is the time.
Given that the amount of radon has a half-life of 3.8 days, it implies that the original amount will decay to half of itself in 3.8 days. Therefore, we will have
![\begin{gathered} (1)/(2)R_0=R_0(1-r)^(3.8) \\ (1)/(2)=(1-r)^(3.8) \\ \sqrt[3.8]{0.5}=1-r \\ r=1-\sqrt[3.8]{0.5} \\ r=0.1667 \\ r=16.7\text{ \%} \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/3okwqwohm7y1mgy0df16u4rbozqq4vr7p0.png)
Answer: The daily decay rate is 16.7% per day
Part B
We can find the formula below;

Answer: Option A
Part C
The percentage of radon after 13 days will be gotten with the formula below.

Therefore,

Answer: 9.30%