Answer: 75.6
Step-by-step explanation
Focus on the first row. We don't know where the 7 values of x fall in the interval 50 ≤ x < 60, so the best estimate to pick is the midpoint 55.
We estimate there are seven copies of "55" in the first row.
The midpoints of the other rows are: 65, 75, 85, 95
Multiply each midpoint with the frequency.
- 55*7 = 385
- 65*11 = 715
- 75*12 = 900
- 85*12 = 1020
- 95*8 = 760
Then add up the results.
385+715+900+1020+760 = 3780
Divide this over the total frequency 7+11+12+12+8 = 50 and we get the final answer to be 3780/50 = 75.6
This sample mean is an estimate because those midpoints are estimates of each row. We would need the actual data set to compute the more accurate mean.