Is this another wrong question (example 14.2) from Ballast 5.0 Review Manual..?
Please help me with this cause I believe the answer of this question should be 0.4, not 0.33. Because the W is 24 instead of 20. According to IBC2015 506.3.2 W is the measured to the closest interior lot line (or the building face) that is minimum 20 feet. The question only gives us one distance which is 24 feet and as I understand it expects us to assume that W is not varying along the perimeter. So we don't need to apply Equation 54 of IBC2015 to find the W value.
So the answer should be:
If= [(180/240) 0.25] x 24/30 = 0.4..????
Thanks and Happy Thanksgiving!!

Elif,
in my opinion...
IBC section 506.3.2 seems to say that the value of "W" is 20 (as long as the actual distance is greater than or equal to 20 feet, but less than 30 feet). Remember, distances of over 30 ft. are handled in this way.
If that is the case, Ballast's answer of 0.3 (0.33) would be accurate.
However, according to the IBC 2015 code commentary this interpretation is inaccurate. The value of W is 24 feet based on the weighted average method found in section 506.3.2. Therefore, (((180 ft /240 ft)  0.25)* 24 ft.) / 30 = 0.4 or answer D.
Ballast's interpretation of code would seem to be inaccurate in this case (although you could make the argument to some code officials that this interpretation is acceptable  and win).
As always though, check up on this yourself.
Good catch!
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