12v + 3b = 243
4v + 4b = 204....multiply by -3
-------------------
12v + 3b = 243
-12v - 12b = - 612 (result of multiplying by -3)
-------------------add
-9b = - 369
b = -369/-9
b = 41 <=== each bus holds 41 students
4v + 4b = 204
4v + 4(41) = 204
4v + 164 = 204
4v = 204 - 164
4v = 40
v = 40/4
v = 10 <== each van holds 10 students