Answer:
It will take 41.3 minutes for the element to decay to 40 grams
Explanation:
The amount of element after t minute is given by the following equation:

In which x(0) is the initial amount and r is the rate that it decreases.
Element X decays radioactively with a half life of 9 minutes.
This means that
. We use this to find r. So








So

There are 960 grams of Element X
This means that


How long, to the nearest tenth of a minute, would it take the element to decay to 40 grams?
This is t when
. So








It will take 41.3 minutes for the element to decay to 40 grams