Divide $ \dfrac{15-15r}{15r^2} $ by $ 12r-12 $ to get $ \dfrac{ -5 }{ 60r^2 } $.

Step 1: To divide rational expressions, multiply the first fraction by the reciprocal of the second fraction.

Step 2: Factor numerators and denominators.

Step 3: Cancel common factors.

Step 4: Multiply numerators and denominators.

Step 5: Simplify numerator and denominator.

$$ \begin{aligned} \frac{ \frac{15-15r}{15r^2} }{12r-12} & \xlongequal{\text{Step 1}} \frac{15-15r}{15r^2} \cdot \frac{\color{blue}{1}}{\color{blue}{12r-12}} = \\[1ex] & \xlongequal{\text{Step 2}} \frac{ \left( -5 \right) \cdot \color{blue}{ \left( 3r-3 \right) } }{ 15r^2 } \cdot \frac{ 1 }{ 4 \cdot \color{blue}{ \left( 3r-3 \right) } } = \\[1ex] & \xlongequal{\text{Step 3}} \frac{ -5 }{ 15r^2 } \cdot \frac{ 1 }{ 4 } \xlongequal{\text{Step 4}} \frac{ \left( -5 \right) \cdot 1 }{ 15r^2 \cdot 4 } \xlongequal{\text{Step 5}} \frac{ -5 }{ 60r^2 } \end{aligned} $$