A hard water sample contains $0\mathrm{.}0061\%Mg$ by mass $($in the form of $M{g}^{2+}$ ions$).$

Part A

The recommended daily allowance of magnesium for those between $19$ and $24$ years old is $0\mathrm{.}4g$ of $Mg.$ How much water $($in grams$)$ contains $0\mathrm{.}4g$ of $Mg?$ Express your answer using two significant figures.