(3) \( 5=\frac{15}{1-r} \)

To solve the equation \( 5 = \frac{15}{1-r} \), start by cross-multiplying to eliminate the fraction. This gives you \( 5(1 - r) = 15 \). Distributing the 5 results in \( 5 - 5r = 15 \). Next, isolate the term with \( r \): \( -5r = 15 - 5 \), which simplifies to \( -5r = 10 \). Dividing both sides by -5, you find \( r = -2 \). Now, you can verify your solution by plugging \( r = -2 \) back into the original equation: \( 5 = \frac{15}{1 - (-2)} \) simplifies to \( 5 = \frac{15}{3} \), confirming your solution is correct!