Calculating rate of fall of a sewer branch line

[sevensages]

Homework Statement

The length of run for a sewer branch line is 36 feet. The difference of elevation is 2 feet. Based on these dimensions, calculate the rate of fall.

Relevant Equations

Rate of fall = difference in elevation/length of run

I am enrolled in a online plumbing course with Stratford Career Institute. This question was on my exam. This question is a multiple choice question. The possible answers are the following: 1/4" per foot of run, 2 inches per foot of run, 1/2 inch per foot of run, and 1.5 inches per foot of run. I answered 1/2 inch per foot of run. So I got this wrong. Stratford Career Institute says that the correct answer is 1.5 inches per foot of run. I don't understand how to get to the correct answer mathematically.

I will put the question on my exam in green font. Here is the question on my exam: The length of run for a sewer branch line is 36 feet. The difference of elevation is 2 feet. Based on these dimensions, calculate the rate of fall.
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Using the textbook, I know that the relevant equation for this question is the following:

Rate of fall = difference in elevation/length of run

So I know that the Rate of fall = 2'0"/36'

rate of fall = 1/18--------------------------

Here is the part that baffles me. How do you get from 1/18 to 1.5 inches per foot of run mathematically?

When I was taking the test, I dismissed the answer of 1.5 inches per foot of run because it seemed to me that 1.5 inches per foot of run would be 54 inches. 1.5 inches per foot of run multiplied by 36 feet of run is 54 inches. 54 inches is 4'6". The exam question said that the difference of elevation is 2 feet, not 4'6".

Again, how do you get from 1/18 to 1.5 inches per foot of run? I am baffled. I need someone to show me the answer to this mathematically rather than only explaining it to me with words.

 

[berkeman]

I agree that 1/18 should be the answer, and I don't see how the listed choices could work. 1/18 is .667"/foot...

In the section of your textbook or other learning resource that discusses rate of fall like this, do all of the examples make sense?

 

[sevensages]

I agree that 1/18 should be the answer, and I don't see how the listed choices could work. 1/18 is .667"/foot...

In the section of your textbook or other learning resource that discusses rate of fall like this, do all of the examples make sense?

Yes.

 

[berkeman]

Okay, good. Can you ping your instructor/TA to ask about this? Please let us know what they say.

 

[Mark44]

I agree that 1/18 should be the answer, and I don't see how the listed choices could work.

I get the same. As some of the answers are given in units of inch/feet, another possible answer would be $\frac 2 3 \frac{\text{inch}}{\text{ft}}$. Since one of the given possible answers is $1.5 \frac{\text{inch}}{\text{ft}}$, it seems possible to me that whoever created the question got the division wrong, using the reciprocal of 1.5.

 

[Beyond3D]

I agree that 1/18 should be the answer, and I don't see how the listed choices could work. 1/18 is .667"/foot...

In the section of your textbook or other learning resource that discusses rate of fall like this, do all of the examples make sense?

Sometimes textbook creators make errors in the books. For example in my shaum's trig textbook the answer key was off by a large amount in "seconds" (the degree listed as $\theta^{\circ}\theta'\theta''$).