What is the total free area in square inches of a 10" x 15" combustion air opening with a 70% free area?

To determine the total free area of a combustion air opening, you should first calculate the area of the opening itself and then apply the percentage of the free area.

In this case, the dimensions of the combustion air opening are 10 inches by 15 inches. To find the total area, you multiply these dimensions:

10 inches x 15 inches = 150 square inches.

Next, since you are given that the free area is 70%, you need to calculate what 70% of this total area is:

0.70 x 150 square inches = 105 square inches.

Therefore, the total free area of the combustion air opening is correctly calculated to be 105 square inches, which corresponds to one of the choices provided. Knowing how to calculate the free area is crucial in fields related to combustion and ventilation, as it impacts the efficiency and safety of combustion processes.