From 2:00 pm to 5:00 pm, Plane A traveled for 3 hours. The total distance it had traveled is the product of its speed and the number of hours which is,
400 mph x 3 hours = 1200 miles
For Plane B, from 2:30 pm to 5:00 pm, it traveled for 2.5 hours and the total distance it had traveled is 812.5 miles. The unknown distance is the hypotenuse of the right triangle such that,
x² = (1200 miles)² + (812.5 miles)²
The value of x from the equation is 1449.19 miles. Thus, the answer is letter A.