SOLUTION: 2 planes leave Newyork AT 10am, one heading for Europe at 600mph and one heading in the opposite direction at 150mph. At what time will they be 900 miles apart?

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

or

In 't' hrs. the first plane travels = miles.
In 't' hrs. the second plane travels = miles.
After 't' hours the distance between the two planes flying in opposite directions = (600t + 150t) = 750t mph.
When, this distance is 900 miles,


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.