Answer:
3.5
Explanation:
First we use the concept of relative speed.
The rates of the 2 planes are 625 mph & 575 mph.
Relative speed will be:
(speed of plane A)+(speed of plane B)
=625+575
=1,200 mph
Distance=4200 miles
Thus the time taken for them to meet will be:
Time=distance/speed
=4200/1200
=3.5 hours
We therefore conclude that the planes met after 3.5 hours.
Hope this helps!