First, you can convert the fraction from mixed to improper to like this
[tex]1\frac{21}{50}=\frac{1\cdot50+21}{50}=\frac{50+21}{50}=\frac{71}{50}[/tex]Now the fraction seen as a decimal is
[tex]1\frac{21}{50}=\frac{71}{50}=1.42[/tex]Finally, to convert decimal to percent move the decimal point to the right 2 places
[tex]1.42\Leftrightarrow142\text{\%}[/tex]