2 jets leave airport at 3pm, one traveling east other west. The westbound jet averages 625km/h and the eastbound jet 825 km/h. At what time will the jets be 725 apart?

Answer 1

You are going to use
D = R x T , (distance = rate x time)

They both travel for t hours
distance of the westbound plane = 625t
distance of the eastern plane = 825t

but 625t + 825t = 725

carry on ....