Final answer:
The trains from Toronto and Montreal, travelling towards each other at speeds of 120 km/h and 90 km/h respectively, are 210 km apart one hour before they meet.
Step-by-step explanation:
The problem presented involves trains leaving Toronto and Montreal at the same time and travelling towards each other, one at 120 km/h and the other at 90 km/h. To find out how far apart the trains are one hour before they meet, we need to consider the relative speed at which they are approaching each other. The sum of their speeds is 120 km/h + 90 km/h, which equals 210 km/h. This is the speed at which the distance between them is decreasing. One hour before they meet, they would have covered a distance equal to their combined speed, since this is the distance they would cover in one hour. Therefore, one hour before they meet, the trains are 210 km apart.