One fire extinguisher be available for every 6000 square feet
Let x the number of fire extinguishers needed for building that has 135,000 square feet.
So, the equation that represents this problem is
                                              6000 x = 135,000
Solve for x
                                          ∴ x = 135,000 / 6000
                                                = 22.5
Rounding to the nearest greater integer
                                         ∴ x = 23
∴ The number of fire extinguishers needed for building = 23