The table shows the number of classes of varying sizes in a school. Using the data, complete the frequency table.
The first table shows us that there were four classes of size 16: two classes had 17 pupils; three classes had 18; five classes had 19 pupils; one class had 20; and four classes had 21 pupils.
The first column of the second table wants us to work out the number of classes of size 16 or 17. There were four classes of size 16. And there were two classes of size 17. Four plus two is equal to six. Therefore, there were six classes of size 16 to 17.
We need to use the same method to work out the number of classes of size 18 to 19. There were three classes of size 18 and five classes of size 19. We need to add three and five. Three plus five is equal to eight. Therefore, there were eight classes of size 18 to 19.
Finally, we need to add one and four as there is one class of size 20 and there are four classes of size 21. One plus four is equal to five. Therefore, there are five classes of size 20 to 21. The three answers are six, eight, and five, respectively, giving us a total of 19 classes in the school.