Question

An element with mass 140 grams decays by 21.6% per minute. How much of the element is remaining after 20 minutes, to the nearest 10th of a gram?

Answer 1

Solution.

The sample decays by 21.6%

 each minute, indicating that the mass at the beginning of the next minute would be 1−21.6%=78.4% the mass at the beginning of this minute.

This process repeats for seven times in the 20 minutes.

That is $140g×(1−21.6\%)^{20}=1.1g;$

Answer: $1.1g.$