We have the following equation.

Where m is the number of feet in a mile.
Let us subtract 20,000 from both sides of the equation then simplify

The negative sign cancels out and the equation becomes

Finally, we can apply the Division Property of Equality which states that when we divide both sides of the equation with same number then the equation remains valid.
Let us divide both sides of the equation by 1.8939 then simplify

Therefore, there are 5280 feet in a mile (rounded to nearest whole number)