When solving for a variable, you want to get the variable to have an equal relationship with a constant. In other words, we want to isolate the variable to one side of the equals sign using logic and Algebraic properties.
Statement: Reasoning:
1.) D=c-s/n [Given]
2.) D•n=c-s/n•n [Multiplicative Property of Equality]
3.) D•n/D=c-s/D [Division Property of Equality]
4.) n=c-s/D [Defined variable]