Let v and b represent the numbers of students a van and a bus can carry, respectively. The given information can be expressed as two equations in these two unknowns:

A system of linear equations such as these can be solved many ways. Here, we observe that the coefficient of v in the second equation is double that in the first equation. We can subtract the second equation from twice the first equation and v will be eliminated, leaving an equation in b that we can solve easily.

A van can carry 7 students.
A bus can carry 20 students.