If $75\%$ of the students in a school are boys and number of girls is $420$ , the number of boys is

Percentage of boys$=75\%$

Percentage of girls$=100-75=25\%$

$25\%\text{of}x=420$

$\frac{25}{100}\times x=420$

$x=420\times 4=1680$                                                                     

So number of boys$=75\%\text{of}1680$

$=\frac{75}{100}\times 1680$

$=3\times 420$

$=1260$