a, You can construct the frequency table with the conditions follow:
1. x∈(0;100]
2. x∈(100;200]
3. x∈(200;300] (there are a famillies)
4. x∈(300;400] (there are b famillies)
5. x∈(400;500]
b, You must know the number of famillies whose monthly utilities bills are x∈(200;400], that the sum of famillies of table 3 and table 4.
So the result is: [(a+b)/35]*100%
