The first question 6) requires us to see how many "events" lies in the 4th class, which is 35 in this case. Then the answer for 6) is 35.
Next, we have to sum the all heights of the rectangles, which are the total dogs weighed as follows:
Hence the answer for 7) is 105
Finally, the midpoint of a class is the half of the x-coordinate. The 4th class is for dogs which weight between 30 and 39, so:
Then the answer for 8) is 34.5