Question 1125918
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Think of f as an actual object, say it's a flute. If we have 1 flute, then we can simply write f or 1f. Having 2 flutes means 2f. Three flutes would be written in shorthand as 3f. And so on.


For our equation,
7f = 7 flutes
6f = 6 flutes


Writing the expression 7f-6f means "we had 7 flutes, and then we took away 6 flutes". So that means we end up with 1 flute or 1f or just f. This property is called "combining like terms". Like terms have the same variable to allow for such combination.


Therefore, 7f-6f = 13 turns into 1f = 13 or <b><font size=4 color=red>f = 13</font></b>


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Let's check the answer. 


Replace every copy of 'f' with 13 and use PEMDAS to simplify the left side


7f-6f = 13
7*13-6*13 = 13 .... every f replaced with 13
91 - 78 = 13
13 = 13 
We end up with a true equation, so that confirms that <b><font size=4 color=red>f = 13</font></b> is the proper answer. 
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