Write down the amount of the drug in the bloodstream at $t = 0$.

Calculate the amount of the drug in the bloodstream after 3 hours.

Use your graphic display calculator to determine the time it takes for the amount of the drug in the bloodstream to decrease to $0.333{\text{ mg}}{{\text{1}}^{ - 1}}$.

$1.2{\text{ (mg}}\,{{\text{l}}^{ - 1}}{\text{)}}$     (A1)     (C1)

[1 mark]

$1.2 \times {(0.87)^3}$     (M1)

Note: Award (M1) for correct substitution into given formula.

$= {\text{0.790 (mg}}\,{{\text{l}}^{ - 1}}{\text{) (0.790203}} \ldots {\text{)}}$     (A1)     (C2)

[2 marks]

$1.2 \times {0.87^t} = 0.333$     (M1)

Note: Award (M1) for setting up the equation.

     (M1)

Notes: Some indication of scale is to be shown, for example the window used on the calculator.

     Accept alternative methods.

$9.21$ (hours) ($9.20519…$, 9 hours 12 minutes, 9:12)     (A1)     (C3)

[3 marks]