29 children rode their bikes to school. after some of the children rode home, there were 8 children with bikes still at school. how many children rode their bikes home?

Subtract how many students are still at the school from the amount of students that went to school in all:

29 students in all - 8 left = how many students went home

So this means:

29 students - 8 students = 21 students that went home

The answer is 1 because you have to flip the sign when dividing by negative number and 3