6v + 3b = 123.....multiply by 4
7v + 4b = 156....multiply by -3
-------------------
24v + 12b = 492 (result of multiplying by 4)
-21v - 12b = - 468 (result of multiplying by -3)
-----------------add
3v = 24
v = 24/3
v = 8
7v + 4b = 156
7(8) + 4b = 156
56 + 4b = 156
4b = 156 - 56
4b = 100
b = 100/4
b = 25
so a bus (b) holds 25 students and a van (v) holds 8 students