1. The speed of the sidewalk: 24 km/hour, with a round-trip distance of 24 km as an example.
2. Loading time of concrete mixer truck: 1/6 hour/car for HZS120 machine, 7/30/hour for HZS90 machine.
3. Unloading time of concrete mixer truck: 5 minutes.
4. It takes 1.25 hours for the HZS120 machine and 1.32 hours for the HZS90 machine to complete one vehicle of concrete transportation.
5. The number of vehicles that can be transported within 6 hours: HZS120 machine 6×1/1.25≈5 (vehicle), HZS90 machine 6×1/1.32≈4 (vehicle).
6. To finish irrigating the beam of 320 m3 concrete within 6 hours, the number of vehicles needed to be transported: 320×1/6.5≈50 (vehicles).