To solve the above question we use the concept of relative speed. The rates of the two planes are 625 mph and 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