Direct & Indirect Proportions

KWF

Junior Member
Joined
Feb 19, 2010
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The answer to the following can be determined by using a direct proportion:

If 12 books cost $120, what is the cost for 6 books at the same cost per book?

Solution:

(books) 12/6 = (cost)$120/?
Cross multiply:
12 X ? = 6 X 120
? = $60.00

Can an indirect proportion be used to determine the same answer if some changes are made to some of the information? The solution requires a unit which is the reciprocal of one in the original question.
 
KWF said:
.....if some changes are made to some of the information?
What's that supposed to mean? :shock:
Post original problem IN FULL.
 


I'm not sure what an indirect proportion is.

You can certainly write different proportions.

12/6 = 120/?

12/120 = 6/?

6/12 = ?/120

120/12 = ?/6

They all lead to ? = 60.

 
".....if some changes are made to some of the information?
What's that supposed to mean? "

The units or a unit in the proportion need(s) to be changed to the reciprocal, but I do not know which units or unit to invert or how to do this. Once the reciprocals are know, the solution now can be solved by using an indirect proportion. This is all that I know!


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"I'm not sure what an indirect proportion is.

You can certainly write different proportions."

12/6 = 120/?

12/120 = 6/?

6/12 = ?/120

120/12 = ?/6



The above example is a direct proportion. As the number of books increases or decreases so does the cost or price. For example, if 12 books cost $120 then 6 book will cost less, $60 in this example.

An indirect proportion is one in which one unit increases the other decreases. For example, if 3 people can rake a yard in 6 hours, how long will it take 6 people? As you can see, it will take less time since the number of people increased.

Indirect proportion solution. Do not set the people and hours next to each other in the proportion.

(people) 3/6 = (hours) ?/6 Cross multiply and the answer should be 3 hours. 6 X ? = 18

Check online for other examples and explanations better than mine.

Thanks!
 
Ahh .. I think we generally call it "inversely proportional".

The problem at hand cannot be expressed as "inverse proportion" - because the inverse units (1/book) will not make sense.

As you had suspected .....
 
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