Question 75591
Let the planes be 900 miles apart after 't' hours from start.


{{{SPEED = DISTANCE/TIME}}} or {{{DISTANCE = SPEEDxTIME}}}


In 't' hrs. the first plane travels = {{{600*t}}} miles.
In 't' hrs. the second plane travels = {{{150*t}}} miles.
After 't' hours the distance between the two planes flying in opposite directions = (600t + 150t) = 750t mph.
When, this distance is 900 miles, 
{{{750t = 900}}}
{{{t = 900/750 = 6/5}}}
So the reqd. time vis t = 1.2 hrs. = 1.2*60 min = 72 min = 1 hr 12 min.
Thus the planes will be 900 miles apart at 11:12 am.