A UNIT MULTIPLIER is a special kind of FRACTION.  Here are three
things you need to know about fractions in order to make unit 
multipliers and use them to change from one unit to another:: 

1. You know that when you multiply a fraction by 1, you don't
   change its value.

2. You know that any fraction whose numerator is the same as
   its denominator is equal to 1.

3. You know that when you multiply fractions, if there is a numerator
   factor and a denominator factor that are the same, you can cancel 
   them out.

4. Unit multipliers are special kinds of fractions that equal
   to one.

To make a unit multiplier you need to know a conversion fact.
We know the conversion fact:  60 minutes = 1 hour.

There are two unit multiplier (fractions) you can make with 
"60 minutes = 1 hour"

These two unit multipliers are  and 

They both have value 1.  That's because their numerator and denominator 
are equal in size.  They don't look the same but we know they are the
same, because 60 minutes and 1 hour are the same.

Now let's do the problem.

Begin by making 300 minutes into a fraction by putting a 1 under it:



Now we have to decide which of these two unit fractions that we can make 
with "60 minutes = 1 hour" we will use to multiply   by.

We notice that  has "minutes" in the numerator.  We
want to get rid of "minutes" and have only "hours", so we choose the unit
multiplier (fraction) that has "minutes" in the denominator so that the
"minutes" will cancel out.  So we use the unit multiplier fraction  
because it has minutes in the denominator.  So we have:



Then we cancel the words "minutes"!  Yes, we can cancel words just like we 
can cancel numbers.  So we cancel the words "minutes":



Now the words "minutes" are gone and all we have are



which is



And since 300÷60 = 5, the answer is

5 hours.

Edwin