Although he hasn’t done division, he will have done his times tables.
So, on Monday, he served a group of families of a certain size (the size of the group not the size of the families!). On Tuesday, he served two more groups of people, so in all he served three groups of families.
What is the size of the group? How many families in the group? In other words, three times what equals 24?
We get to three times eight. Bingo! So the group size is 8 families. 8 on Monday, two times eight on Tuesday is sixteen. 16 families on Tuesday.