1 mL =[tex]10^{-3}[/tex] L
so $155$ mL =$155\times10^{-3}$ L
Now, $1$ gallon = $3.785$ L , or $1$ L= $\frac1{3.785}$ gallons
so $155\times10^{-3}$ L= $155\times10^{-3}\times \frac{1}{3.785}=40.95\times 10^{-3}$ gallons
and this is for each day, so just divide by 1 (day) to get the ratw gallons/day
i.e. $40.95\times 10^{-3}$ gal/day
