php float comparison not working

I want to change the students marks to GP so I have code like this

$mark=80;    
if($mark>=84.50 && $mark<=100){$GP = 4;}
if($mark>=79.50 && $mark<=84.49){$GP = 3.67;}
if($mark>=74.50 && $mark<=79.49){$GP = 3.33;}
if($mark>=69.50 && $mark<=74.49){$GP = 3;}
if($mark>=64.50 && $mark<=69.49){$GP = 2.50;}
if($mark>=59.50 && $mark<=64.49){$GP = 2;}
if($mark>=54.50 && $mark<=59.49){$GP = 1.50;}
if($mark>=49.50 && $mark<=54.49){$GP = 1;}
else { $GP = 0; }
echo $GP;

Expected output here is 3.67 but here I am getting 0. Can anyone explain why I am getting 0?