Step 1: Understanding the Concept:
The distance between the first minima on either side of the central maximum is $2y_1 = \frac{2D\lambda}{w}$.
Step 2: Detailed Explanation:
Given $2y_1 = 5$ mm, $D = 80$ cm = $0.8$ m, $\lambda = 6000 \times 10^{-10}$ m.
So, $w = \frac{2D\lambda}{2y_1} = \frac{2 \times 0.8 \times 6000 \times 10^{-10}}{5 \times 10^{-3}} = 0.192 \times 10^{-3}$ m = $0.192$ mm.
Step 3: Final Answer:
The slit width is $0.192$ mm.