If 10% of a radioactive material decays in 5 days then the amount of the original material left after 20 days is approximately:

Disintegrating % in given time = 10

Surviving % in the given time = 90

Surviving fraction in the given time  $f=\frac{90}{100}=0.9$

Timeperiod for given fraction of material to survive  $t_f=5days$

Time for which we need to find survival amount  $t=20days$

Number of timeperiods in the required time $n=\frac{t}{t_f}$

The number survived in the given time $N=N_0{f}^n$

Fraction of amount survived  $\frac{N}{N_0}=f^n={0.9}^4=0.656$

Percentage survived in the given time $\frac{N}{N_0}\times 100=f^n\times 100=65.6\%$