To construct a tally chart for the groups 1-5, 6-10, 11-15, 16-20 and work out the frequency of each group, we can do the following:
1-5: ||||| (5)
6-10: ||||| ||||| (10)
11-15: ||||| ||||| ||||| (15)
16-20: ||||| ||||| ||||| (15)
In the tally chart, each vertical line represents one data point. So, for example, the group 1-5 has 5 data points, which we represent with five vertical lines (|||||).
To work out the frequency of each group, we simply count the number of tally marks in each group. The frequency is the number of data points that fall within each group. Based on the above tally chart, we have:
1-5: 5
6-10: 10
11-15: 15
16-20: 15
Therefore, the frequency of each group is 5 for 1-5, 10 for 6-10, 15 for 11-15, and 15 for 16-20.