Two airplanes leave the same airport. one heads north, and the other heads east. after some time, the northbound airplane has traveled 44 miles. if the two airplanes are 55 miles apart, how far has the eastbound airplane traveled?

So, We know that two airplanes leave the same airport.
One goes north. 
The north airplane has traveled 44 miles?

North:(44m/ph)
East:(?)

So we know that they're 55 miles apart. 

Which this makes the question pretty easy to solve. 

Well only if you understand it.

North:(44m/ph)

East:(55m/ph ahead of 44m/ph)
55 + 44 = 99m/ph

Now we know that the East plane has traveled 99 miles.
We found out this by adding 55 miles to the 44 miles that the north plane had traveled. The reason of this work is because we know that the East plane is 55 miles apart from the north plane. Then it got easy, all we needed to do is add 55 to 44. Which got us the answer of 99 miles.

(99m/ph)___(99 miles)