Question

An airplane travels at an unknown elevation before it descends 3 miles to an elevation of 15 miles. Find the elevation of the plane before its descen.

Answer 1

18 miles

Explanation:

Let's call it's initial elevation x. Because it descends 3 miles, we now have x - 3. The question says it is now 15 miles above sea level, or x - 3 = 15, and adding 3 to both sides shows that x = 18 miles.