Janie receives an allowance of $3 per week. In addition, she can earn $2 for each chore she does. This week, she wants to earn enough money to buy a CD for $13. Janie can do fractions of chores.
Let us say that the number of weeks is "x" and the number of chores is "y".
If we state an algebraic expression for Janie's allowance it would be
3x+2y=13
However, there are two variables, hence many answers. To solve this problem, there must be more information about the relation between x and y.