Making the fractions in a proportion

"How do you know how to make the fractions in a proportion?
When making them, how do you know where each number goes making the fraction, like which ones go on top of the fraction?"

Well, actually you can choose which quantity will go on top; the proportion WILL work either way!

But, sometimes people are used to always putting certain quantity on top and certain on the bottom. For example, if the question is about speed and the unit is "miles per hour", that tells you that miles go on top, and hours on bottom, because "per" means division (the fraction line).

However, you could still solve the proportion by putting hours in the numerator of the fractions and miles in the denominator, and the calculation will turn out alright.

Or, if the question is about "dollars per pound", then dollars go to the numerator and pounds in the denominator.

Let's look at this problem for example:

A car drives on constant speed. It can go 80 miles in 90 minutes. How long will it take for it to travel 100 miles?

You can make both fractions to be




Let's try the first way:

80 mi 100 mi
-------- = ---------
90 min x

To solve, cross multiply and you get 80x = 100 * 90, and then x = 900/8 = 112.5 minutes.

The other way it will be

90 min x
-------- = --------
80 mi 100 mi

To solve, cross multiply and you get 80x = 90 * 100

You see, the final equation ends up being the same, no matter which
quantities were on top of the fractions.

HOWEVER, one way is wrong: that is if you but "miles" on top in one
fraction, and "minutes" on top in the other... then you'll get it wrong:

90 min 100 mi
-------- = -------
80 mi x min

=> 90x = 100* 80 (WRONG)


Mr. C said…
Interestingly, the proportion also works if you set it up this way:

100 mi x min
________ = ________

80 mi 90min

where, x is the time it takes to travel 100 miles.

However, this only works if you keep the related parts the same on top and bottom. The first distance and time on the bottom and the second distance and time on top.

For this proportion I put the second relation on top because I knew I had to solve for the unknown time, and it relatively saves a step in the Algebra, but I could have easily set it up as

80 mi 90 min
________ = ________

100 mi x min
Anonymous said…
However you guys are totally right but i think kids will also need to know how would you figure that out in a word problem! like Mrs.Flores has 7 boys and 10 girls in her class.What would be the Proportion for this problem?

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