Aschool bus has a mass (including the driver and passengers) of 1.64 times 10^4 kg and is driving north at a speed of 15.2 km/h. another school bus whose empty mass is 1.578 times 10^4 kg is moving south at a speed of 12.2 km/h. assuming an average passenger mass of 64.8 kg, what is the minimum number of passengers (including the driver) of the southbound bus so that the total momentum of the two buses is directed south?