The planes will pass each other in 1.14 hours.
Why?
To solve the problem, we must remember that since both planes are heading toward each other, the speed to the calculations will be their combined speeds.
The speed will be:
[tex]Speed=PlaneA+PlaneB\\\\Speed=620mph+780mph=1400mph[/tex]
Now, calculating the time, we have:
[tex]x=v*t\\\\1600mi=1400mph*t\\\\t=\frac{1600mi}{1400mph}=1.14hours[/tex]
Hence, the planes will pass each other in 1.14 hours.
Have a nice day!