Question

If you start with 500.0 g of a radioisotope with a half-life of 3.0 days, how much of the original isotope will still be in the sample after 18 days?

Answer 1

Half life means it decrease by half every 3 days
250 - 3 days
125 - 6 days
62.5 - 9 days
31.25 - 12 days
15.625 - 15 days
7.8125 - 18 days
7.8125 is your final answer.

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Answer 2

After 3 days  = 500g becomes 250g
After 6 days  = 250g becomes 125g
After 9 days  = 125g becomes 62.5g
After 12 days = 62.5g becomes 31.25g
After 15 days = 31.25g becomes 15.625g
After 18 days = 15.625g becomes 7.8125g

After 18days, you would have about 7.813g left.