Distance (in km) covered in a shift by auto drivers of a Guwahati stand:
0-10: 8, 10-20: f, 20-30: 12, 30-40: 6, 40-50: 4.
The mean distance is 22 km. Find the missing frequency f.

Class marks 5, 15, 25, 35, 45. $\sum f_i{x}_i=40+15f+300+210+180=730+15f$ and $\sum f_i=30+f$. Setting $\frac{730+15f}{30+f}=22$ gives $730+15f=660+22f$, so $7f=70$ and $f=10$. "35" comes from using the class boundary 20 as the class mark of 10-20 instead of 15; "70" is $7f$ left undivided; "3" comes from forgetting the missing class's own term $15f$ in the numerator.