This is a graph represents the number of times a week the students in a class walk to school
A question asked is "In total, how many times do all the boys in the class walk to school per week?"
I don't even understand the question to be honest. What is being asked and what is the answer?
Have a look at the first entry. It says that $3$ boys never walk to school. This then means that $0$ times do all the boys in the class walk to school. You could not have entries for $0$ walks if it ever happened that all the boys walked to school.