In this question, it is given that
It is recommended that one fire fire extinguisher be available for every 6000 square feet in a building .
And X is the number of fire extinguishers needed for building that has 135 ,000 square feet area .
We have to find the value of X.
And for that we have to divide 135000 by 6000,that gives

So approx 23 fire extinguishers are needed for an area of 135000 square feet .