The amount of radioactive element remaining, r , in a 100 -mg sample after d days is represented using the equation r is equal to 100 times 1 half raised to the d over 5 power What is the daily percent of decrease?

The sample retains half its amount every 5 days, so the fraction remaining after 1 day is (1/2)^(1/5), which is about 0.8706. The daily decrease is 1 − 0.8706 = 0.1294, or about 12.94%. Therefore, the daily percent of decrease is 12.94%, choice B.

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