Kali left school and traveled toward her friend’s house at an average speed of 40
km/h, matt left one hour later and traveled in the opposite direction with an average speed of 50 km/h. find the number of hours matt needs to travel before they are 400 km apart.

how can i figure out an equation to solve this problem?

Matt needs to travel 3 hours. 
If 40km=1 hour
   400km=? But there are two distances
200/40*1=5 hours
Therefore Kali needs to travel 5 hours to be 200 km away from the house.
If 50km=1 hour
200km=?
200/50*1hr=4hrs Therefore Matt needs to travel 4hrs to be 200 km away from the house. But Kali traveled 1hr earlier than Matt. 4hrs-1hr=3hrs
Therefore Matt has to travel for 3 hours to be 400km away from Kali.