Answer
$8.\overline3\ hr$
Work Step by Step
Find the total number of drops.
$2\ L\times\frac{1000\ mL}{1\ L}\times\frac{10\ drops}{1\ mL}=20000\ drops$
Divide the number of drops by the flow rate to find the time.
$20000\ drops\div\frac{40/ drops}{1\ min}=20000\ drops\times\frac{1\ min}{40\ drops}=500\ min$
Convert to hours.
$500\ min=500\ min\times\frac{1\ hr}{60\ min}=8.\overline3\ hr$
