For a certain radioactive substance, it is observed that after 4 hours, only 6.25% of the original sample is left un-decayed. Choose the only wrong statement from the following:

$6.25\%=\frac{6.25}{100}=\frac{1}{16}=\frac{1}{{\left(2\right)}^4}$. 

Hence 4 hours are equal to 4 half lives. Therefore, the half life of the substance is 1 hour.

 The decay constant = $\frac{\ln \left(2\right)}{\mathrm{half} \mathrm{life}}$= $\ln \left(2\right)$ per hr, 

Mean life = $\frac{1}{\mathrm{decay} \mathrm{constant}}$

                = $\frac{1}{\ln \left(2\right)}$ hour = $\frac{1}{0.693}$hr

After further four hours (8 hours from starting), the amount of substance left would be:
$\frac{\mathrm{N}}{{\mathrm{N}}_0} = \frac{1}{2^8} = 0.0039$
Percentage of amount of substance left would be $\left(\frac{\mathrm{N}}{{\mathrm{N}}_0} \right)\times 100 \% = 0.0039 \times 100 \% = 0.39 \%.$