The depression in freezing point of the solution can be written as follows,
ΔTf=i⋅Kf⋅m
molality (m) =\frac{w/w%}{\mathrm{Molar}\mathrm{mass}\mathrm{of}\mathrm{solute}}\times \frac{1000}{\mathrm{mass}\mathrm{of}\mathrm{solvent}\mathrm{in}\mathrm{grams}}
⇒ΔTf=2.67×1.8×9838×621000
⇒ΔTf=30.05
∴ Freezing point of solution at which battery freeze=243K