I think I can.
Let's do it together:
Let's call the number of buses 'B', and the number of vans 'V'.
(Pretty clever so far, don't you think ?)
OK. What do we know ?
-- Each bus holds 51 passengers. The number of passengers in 'B' buses is 51B.
-- A van holds 10 passengers. The number of passengers in 'V' vans is 10V.
-- The total number of passengers ... (51B + 10V) ... is 142.
-- The total number of vehicles ... (B + V) ... is 6, because there are 6 drivers.
Can you make a system of equations out of that information yet ?
How about this:
51B + 10V = 142
B + V = 6
I really think you can handle it from here.
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Multiply the 2nd equation by 10 :
51B + 10V = 142
10B + 10V = 60
Subtract the 2nd equation from the 1st one:
41B = 82
Divide each side by 41: B = 2 buses
2 buses . . . . 2 drivers . . . . 2 x 51 = 102 passengers
4 vans . . . . . 4 drivers . . . . . 4 x 10 = 40 passengers
6 vehicles . . . 6 drivers . . . . . 102 + 40 = 142 passengers