Number of days required for a sample to decay from 10 milligrams to 1 milligram....

Formula used:
Radio-active decay of a sample is given by ${\displaystyle N=N_0{e}^{-kt}}$
Where, ${\displaystyle N_0}$ is the initial weight of the sample.
Calculation:
At ${\displaystyle t=0,N=N_0=10}$
At ${\displaystyle t=23,N=5}$
Substitute in ${\displaystyle N=N_0{e}^{-kt}}$
${\displaystyle 5=10e^{-k\left(23\right)}}$
${\displaystyle k=\frac{\ln 2}{23}=0.03}$
To calculate the number of days to decay the sample to 1 milligram, Substitute ${\displaystyle N=1\in N=N_0{e}^{-kt}}$
${\displaystyle 1=10e^{-0.03t}}$
${\displaystyle t=\frac{\ln 10}{0.03}=76.4}$ days.
Conclusion:
Using the formula of radioactive decay, the number of days can be calculated for a sample to decay from 10 milligrams to 1 milligram.