For combining like terms: it is 7z
A unit rate means for one, so in this case they mean for one pound of the horse feed which brand sells the highest. You would divide the cost into the number of pounds for each brand. Now, compare all the brands and see which one is highest.

Write three ratios equal to 

This ratio can be represented as 
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Equivalent/Equal ratios<em> - Definition of Equivalent Ratio A ratio can be represented as a fraction. The concept of an equivalent ratio is similar to the concept of equivalent fractions. A ratio that we get either by multiplying or dividing by the same number, other than zero, to the antecedent and the consequent of a ratio is called an equivalent ratio.</em>
10+2(x)=5(x-7)
10+2(x)=5(x)-35
35+10=5(x)-2(x)
45=3(x)
45/3=x
x=15