What is the calculation for the capacity of a 5-inch vent after applying the 10% reduction, given a base capacity of 122 MBtu/h?

What is the calculation for the capacity of a 5-inch vent after applying the 10% reduction, given a base capacity of 122 MBtu/h?

• A. 122 × 0.90 = 110 MBtu/h
• B. 122 ÷ 0.90 = 135.56 MBtu/h
• C. 122 × 1.10 = 134.2 MBtu/h
• D. 110 × 0.90 = 99 MBtu/h

Answer: (A)

Reducing by 10% means you keep 90% of the original capacity. So you multiply the base by 0.90: 122 × 0.90 = 109.8 MBtu/h, which is typically written as about 110 MBtu/h. This is why the correct approach is to apply the 0.90 multiplier to the base value. Why not other ways? Dividing by 0.90 would increase the capacity (135.56 MBtu/h), not reduce it. Multiplying by 1.10 would also increase (134.2 MBtu/h). Using 110 and then applying 0.90 would not reflect the base value given.

Reducing by 10% means you keep 90% of the original capacity. So you multiply the base by 0.90: 122 × 0.90 = 109.8 MBtu/h, which is typically written as about 110 MBtu/h. This is why the correct approach is to apply the 0.90 multiplier to the base value.

Why not other ways? Dividing by 0.90 would increase the capacity (135.56 MBtu/h), not reduce it. Multiplying by 1.10 would also increase (134.2 MBtu/h). Using 110 and then applying 0.90 would not reflect the base value given.