<PRE><B>Question 776038</B>
Let the required time be T hours.


In T hours the distance travelled by the plane flying East is 560T km.
In T hours the distance travelled by the plane flying West is 640T km.


Distance between the two planes at time T is (560 + 640)T = 1200T km.


By problem, 1200T = 2100

So, T = 2100/1200 = 7/4 = 1.75 hours.</PRE>