Answer:
750
Step-by-step explanation:
Pick one of the schools as the host. There are C(5, 2) ways to select the two representatives from that school and C(5, 1) ways to pick a representative from each of the other schools. So once we have selected a host school, there are 10*5*5=250 ways to pick the representatives. However, any of the three schools can be the host, so we need to multiply 250 by 3 to get 750 ways.