Copies of a newspaper sold per day by 100 vendors of a district:
0-100: 12, 100-200: 22, 200-300: f, 300-400: 20, 400-500: 16.
Find the mean number of copies sold per vendor.

The missing frequency comes from the total: $f=100-(12+22+20+16)=30$. Class marks 50, 150, 250, 350, 450 give $\sum f_i{x}_i=600+3300+7500+7000+7200=25600$, so the mean is $\frac{25600}{100}=256$ copies. "250" simply quotes the class mark of the middle class; "206" uses the lower limits as $x_i$; "306" uses the upper limits.