Explanation:
Let x be the elevation from which the plane took off.
The plane climbed 20,000 feet to reach an elevation of 21,250 feet.
So, the difference between the two elevations is:
21,250 - 20,000 = 1,250
This difference is equal to the elevation from which the plane took off minus the initial elevation of 20,000 feet:
x - 20,000 = 1,250
Solving for x:
x = 1,250 + 20,000
x = 21,250
Therefore, the plane took off from an elevation of 21,250 feet.