1.

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

Answer»

0.6
0.65
0.7
0.75

Solution :After 5 days, 10% decayed then 90% left. After 10 days 10% of 90% decayed, then 81% left.
After 15 days, 10% of 81% decayed then 72.9% left.
After 20 days 10% of 72.9% decayed then 65% left.


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