21/40 = 0.525 0.525 = 52.5%

[latex]p-the \ percentage \\ \\ p \% \ of \frac{21}{40} =\frac{p}{100}*\frac{21}{40} \\ \\ p=\frac{21*100}{40} \\ \\ p=\frac{2100}{40} \\\\ \boxed{p=52.5}[/latex]