Frequency is the number of occurrences of a repeating event.
Here we have the events:
shopping time between occurences frequency
18 to 22: 20, 18 2
23 to 27: 27, 24, 26, 24, 24 5
28 to 32 29, 31 2
33 to 37 37, 34, 35, 33, 36, 32 6
5.6m questions
7.3m answers