The truck travels at v (km/h).
Fuel is consumed at 6 + \frac{v^{2}}{50} (L/h). But as fuel is 1 ($/L), we know that the cost of fuel per hour is 6 + \frac{v^{2}}{50} ($/h).
Pay for the drivers is 80 ($/h) - because it's 40 each.
Now, 1000 km travelled at v (km/h) is done in: \frac{1000}{v}...