Problem $3$: Reverse osmosis $(RO)$ is being used more widely for treating groundwater. $RO$

treatment can produce water that essentially has no hardness. A $3$ MGD RO water treatment

plant $($WTP$)$ treats groundwater containing $150m\frac{g}{L}$ of hardness as CaCO$3.$ The treated water

from the RO WTP will be blended with untreated groundwater to achieve a final hardness of $50$

$m\frac{g}{L}$ as CaCO. The amount $($MGD$)$ of untreated groundwater must be blended is most nearly:

$($A$)0\mathrm{.}5$

$($B$)1\mathrm{.}9$

$($C$)1\mathrm{.}2$

$($D$)1\mathrm{.}5$