Present Height plus decreased height equal to original height.

The equation represents the situation is, .

Apply subtraction property of equality: if , then .

         (Subtract 450 from each side)

          (Apply commutative property of addition: )

                         (Apply Additive inverse property:)

                         (Apply additive identity property:)

                                          (Subtract:)

Check:

To check the solution, substitute in original equation.

                                     (Add: )

Compare the values, the equation is true. The solution is .

Decrease in the height of the pyramid is 31 ft.