• # question_answer 49) If 10% of a radioactive material decays in 5 days, then the amount of the original material left after 20 days is approximately A)  $60%$                        B)  $65%$C)  $70%$                        D)  $75%$

Let initial amount be 100 gm.  disintegrated Left $100gm\,\xrightarrow{5\,days}$$\frac{100\times 10}{100}$ 10 90 $90\,\xrightarrow{Next\,5days}$$\frac{90\times 10}{100}$ 9 81 $81\xrightarrow{Next\,5\,days}$$\frac{81\times 10}{100}$ 8.1 $\approx \,73$ $73\xrightarrow{Next\,5\,days}$$\frac{73\times 10}{100}$ $7.3$ $\approx \,65$