15x means that $15 was charged for each exercise classes taken above the planned amount
55y means that $55 was charged for each massages taken above the planned amount
15x + 55y + 20 represent the total charges in one month.
An equation is an expression that shows the relationship between two or more numbers and variables.
An independent variable is a variable that does not depend on other variables while a dependent variable is a variable that depends on other variables.
If x is the number of exercise classes taken above the planned amount and y is the number of massages taken above the planned amount, hence:
15x means that $15 was charged for each exercise classes taken above the planned amount
55y means that $55 was charged for each massages taken above the planned amount
15x + 55y + 20 represent the total charges in one month.
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