the concentration of a certain substance in our organism is given by the model Q9t) = Q(subscript 0) e^-0.1960t
a) what quantity of substance is present in the organism of this individual 3 hours after ingestion?
b) after how many hours will there only be 10% of this substance ingested by the individual?
Q(t) = Q0 e^-0.1960t
where t=elapsed time in hours (Check, not mentioned in the question).
So after three hours,
Q(3) = Q0 e^(-0.1960*3)=?
Q(t) = 0.9Q0
or
e^(-0.1960t)=0.9
Take natural log:
-0.1960t = ln(0.9)
t=ln(0.9)/(-0.1960)=?
I'll leave it to you to work on your calculator.