This question 5 is from chapter 6 (C++ for engineers and scientists 4th edition by Gary J. Bronson.

I want this question to be solved as simple as possible.

The equivalent diameter of a circular duct that provides the same pressure loss asa rectangular duct (see Figure 6.18) is given by the Huebsher equation: a+ 6)0.25 d, is the equivalent diameter (mm or in). a is the rectangular duct width (mm or in). b is the rectangular duct height (mm or in)