Using, Mode =   $l+\frac{f_1-f_0}{{2f}_1-f_0-f_2}\times h$  
where,
Maximum class frequency, $f_1=8$

The class corresponding to the frequency $=20-40$

Lower limit of the modal class, $l=20$

Frequency of class preceding the modal class, $f_0=7$

Frequency of class succeeding the modal class, $f_2=2$                                

$Now,Mode=20+\frac{8-7}{16-7-2}\times 20$

 $Mode=20+\frac{1}{7}\times 20$

$\therefore Mode=22.86$