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              NOT MY WORDS TAKEN FROM A SOURCE!

You start with "5 yards" 

You know 1 yard = 36 inches. 

So you wonder if you should multiply by 1/36 or 36/1. 

The way to figure this out is to keep track of the units. 

You have yards, so you want the conversion factor to be a fraction, with yards in the denominator. So you want 36/1 because that's really 36 inches / 1 yard. 

Now when you multiply: 

5 yards X (36 inches / 1 yard) 

the "yards" units cancel out, leaving inches. The answer is 180 inches. 

When you have a rate, like miles/hour, just makes sure that when the units you want to change are in the denominator, then the conversion factor must have them in the numerator, and vice versa. 

It's really pretty simple -- the units are the key to setting it up easily.

Hope this helped!

:)