Step 1: Recall the formula that relates marginal revenue (\(R_M\)), average revenue (\(R_A\)), and the elasticity of demand (\(\eta\)).
\[ R_M = R_A \left(1 - \frac{1}{\eta}\right) \]
Step 2: Substitute the given values \( R_A = 50 \) and \( \eta = 5 \) into the formula.
\[ R_M = 50 \left(1 - \frac{1}{5}\right) \]
Step 3: Simplify the expression.
\[ R_M = 50 \left(\frac{4}{5}\right) = 10 \times 4 = 40 \]