How do you write algebraic expressions to model quantities?

To write algebraic expressions to model quantities is you base the algebraic expressions or mathematical values declared in words by the manner of how the sentence illustrates the said problem. For example, one minus a number is the difference of zero shall be 
1. 1 – n = 0. 
Other same examples, John bought 25 apples while Julia bought only 15 from the market. If Niccole bought twice as much as Julia’s, how many was Niccole’s apples? 
1.      You state the problem by the sequence of the said problem and understanding the possible structure. 
2.      Solution will be Julia = 15 apples Niccole = 2Julia   
Hence, 2Julia = 2(15) = 30 apples.