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

Every minute 21.6% or the initial 140 grams disappears.

That means after each minute 78.4% of what was there a minute ago remains (100% - 21.6% = 78.4%).

The function that models this situation is:

M(t) = 140 (0.784)^t (note that when t = 0, M= 140).

To find what remains after 20 minutes:

M(20) = 140(0.784)²⁰ (probably best to do that calculation with a calculator).

Check your result here:

desmos.com/calculator/4oqlxolkpm