We need to find frequency in the region.
[tex]\\ \sf\longmapsto 50\leqslant l[/tex]
We should calculate in it frequencies of below
[tex]\\ \sf\longmapsto 50\leqslant l<60[/tex]
[tex]\\ \sf\longmapsto 60\leqslant l<80[/tex]
[tex]\\ \sf\longmapsto 80\leqslant l<100[/tex]
[tex]\\ \sf\longmapsto 100\leqslant l<150[/tex]
Hence
total frequency:-
[tex]\\ \sf\longmapsto 8+b+c+d[/tex]
