How many grams of KCI would have to be dissolved in 60g ${\mathrm{H}}_2\mathrm{O}$ to give 20% by weight of solution?

As we know,

$\mathrm{\%}\text{ by mass }=\frac{\text{ Mass of solute }}{\text{ Mass of solution }}\times 100$

$20=\frac{\text{ Mass of }\mathrm{KCl}\times 100}{\text{ Mass of }\mathrm{KCl}+\text{ Mass of water }}$

Let, W be the mass of KCl

$\begin{array}{l}\frac{W}{W+60}\times 100=20\\ \Rightarrow W=15\mathrm{g}\end{array}$