Answer
$78 \space g$ of $KNO_3$ will dissolve in 120 g of $H_2O$ at $40 ^oC$
Work Step by Step
1. Use the solubility as a conversion factor to find the amount of $KNO_3$ that will dissolve in 120 g of $H_2O$ at $40 ^oC$
$$120 \space g \space H_2O \times \frac{65 \space g \space KNO_3}{100 \space g \space H_2O} = 78 \space g \space KNO_3$$
