Answer
It takes the planes 1.5 hours to be 1125 miles apart.
Work Step by Step
Let t represent the time it takes the planes to be 1125 miles apart.
The first plane travels at 350 miles per hour, for t hours, so the distance it flies is: 350 $\times$ t = 350t.
The second plane travels at 400 miles per hour, for t hours, so the distance it flies is: 400 $\times$ t = 400t.
The planes fly in opposite directions. To find the time it takes them to be 1125 miles apart, we set up the following equation:
350t + 400t = 1125.
750t = 1125
Divide both sides by 750:
t = 1.5
It takes the planes 1.5 hours to be 1125 miles apart.
