ANSWER
EXPLANATION
Let the number of people riding in each bus be x.
We have that 20 people are riding in 4 vans and there are x people in each van (given that there is an equal number of people in each van).
This means that the product of the number of people in each van and the number of vans must be equal to the total number of people.
That is:
Solve for x:
That is the answer.