Answer
(a) The horizontal asymptote is y=0. As time increases the concentration will practically disappear.
(b) See below.
(c) 5 minutes.
Work Step by Step
(a) If the degree of the denominator is greater than the degree of the numerator, as is the case here, the horizontal asymptote is y=0. As it happens with most things we intake, there will be a spike that prompts the body to stabilize to conditions before the intake.
(b) Used a graphic calculator.
(c) The calculator approximated $t$ as exactly 5 minutes.
