For every 6000 square feet in a building, there is one fire extinguisher.
So for 135000 square feet, the equation is like direct proportion (because the more the square feet, the more fire extinguishers needed).
1 : 6000
x : 135000
Multiply x with 6000, and 1 with 135000.
So, the result is:
6000x = 135000
x = 135000/6000
x = 22.5