Answer:
It'll take approximately 9.1 minutes
Explanation:
The decay of this element can be modeled by the following expression:
m(t) = 540*(1 - 0.179)^t
Where m is the mass of the element in a given minute t. This expression comes from a compounded interest formula, with negative rate, since they will work in the same manner. Therefore to find how long until it reaches 90 g, we need to make m = 90 and solve for t.
90 = 540*(0.821)^t
(0.821)^t = 90 / 540
(0.821)^t = 1/6
ln(0.821^t) = ln(1/6)
t*ln(0.821) = ln(1) - ln(6)
t*ln(0.821) = - ln(6)
t = -ln(6)/ln(0.821) = 9.08452
It'll take approximately 9.1 minutes