Independent variable is the variable that you change.
Dependent variable is the variable you measure that results from or depends on the independent variable.
Since you want "r" to be the independent variable, you isolate/get the variable "q" by itself in the equation: [this is because "r" would be the variable that changes, and "q" would be the variable you find/measure that depends on "r"]
10q - 5r = 30 Add 5r on both sides
10q = 5r + 20 Divide 10 on both sides to get "q" by itself