Step-by-step explanation: look so if you have
 shcool A its 8v+4b=12
and 
School B: 12v+4b= 16 
since both classes used 4 busses we can use elimination by subtracting the A class Equation from the b class equation to solve from v for its value Then, use either equation and the now found value for v to solve for b. 
You should find part of the process to be 
12v+4b-(8v+4b)=12-16 which shows the van holds only a few people even before continuing the solution.
Answer -4