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One number is the base. It's the total amount of something or the measure of something.
Second number is the rate. It is a measure of the part relatively to the whole thing, it's shown like 3%, 23%, 3.75% .
the third number is the part.. it's the amount or the measure of the part. 

you could use the triangle to remember how (PRB) 
.... ... /"\
....../  P  \
..../ R | B \