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Explanation:
Refer to the table below.
The first step is to find the midpoint of each interval.
To do that, we add up the endpoints and divide by 2. The midpoint of the interval from 2 to 6 is 4 because (2+6)/2 = 8/2 = 4. Repeat these steps for the other intervals to find that the remaining midpoints are: 8, 12, 16, 20.
Overall, the midpoints from top to bottom are 4, 8, 12, 16, 20
Note how the midpoints are incrementing by 4 each time, which is the same distance between each successive left endpoint (and right endpoint endpoints as well).
Let's say m represents the set of midpoints which I'll form in the third column. Multiply the frequency column f by each corresponding midpoint m. The result fm forms the fourth column.
Add up everything in that fourth column to get 1140. We then divide that by the sum of the frequencies (100) to get 1140/100 = 11.4 which is the final answer. The average weight is 11.4 pounds.
