For whatever value of N that you are given, you need to see where that value of N lies in respect to the three separate ranges given:
N>or equal 500
For whatever range your value of N lies between, you use the equation that is associated with that range of values.
For instance, if that value of N given is 250, then you would look at your range of values and see that 250 lies between 100 and 500, therefore you would use the equation .09xN to solve for that particular value.
Hope this helps.
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