On the most recent exam on Prof. Xochi's class,

$10$ students earned a score of at least $90\%$
$25$ students earned a score of at least $80\%$
$40$ students earned a score of at least $70\%$
$60$ students earned a score of at least $60\%$

How many students earned a score of at least $70\%$ and less than $80\%$?