Line Plots · Superchallenge problem

A line plot shows how many brothers and sisters each of the $20$ kids in a club has. All the brothers and sisters of every kid are also in the club. Above $0$ there are $3$ X's, above $1$ there are $6$ X's, above $2$ there are $3$ X's, and above $3$ there are $8$ X's. How many different families are in the club?