how do I calculate a number as a percentage of another number

The answer is 62 and some decimal that I do not know when it ends. 

what % of 283 is 224    

You know it's going to have to be division, but which one is the divisor.  

I've been trying to think of a general guideline to determine that. 

I'd use a known percentage as an example.   

Like, What % of 6 is 3.  We know it's 50%, so when the problem is  

phrased like that, you divide 3 by 6.  Since the problem in question   

here is phrased exactly like that, you divide the last by the first.    

                                                           224     

                                                          –––––  =  0.7915           

                                                            283      

I truncated the fraction after four places.  If you want to take it out farther    

than that, you can do your own division and use as much of it as you want.   

Now that we have it as a fraction, all you do is multiply that fraction by 100    

to obtain the proportion stated as a percentage.    

                                                         0.7915 • 100  =  79.15%    

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