According to the question
$E+H=100$,
where;
$E$ = number of students who can speak English,
$H$ = number of a student who can speak Hindi.
There are $10$ students who don’t speak any one of the languages.
$\therefore$ $H+E=90$
$n(E)=20,n(H)=80,n(H\cup E=90)$
$\because$ $n(H\cup E)=n(E)+n(H)-n(H\cap E)$
$\implies$ $90=20+80-n(H\cap E)$
$\implies$ $n(H\cap E)=10$
The number of students knowing both Hindi and English is $10$