Answer
$3.82\;m\;Zn$
Work Step by Step
For ease of this calculation, we will assume 100 g sample. Recall that molality is mol solute / kg solvent.
Find mass of Zn in the sample.
$100\;g\;sample\times\;20\%\;Zn=20.0\;g\;Zn$
Find moles of Zn (molar mass = 65.38 g/mol)
$\frac{20.0\;g\;Zn}{}(\frac{1\;mol}{65.38\;g})=0.306\;mol\;Zn$
Find molality.
$\frac{0.306\;mol\;Zn}{100\;g\;sample}=3.82\;m\;Zn$
