1. **State the problem:** Simplify the expression $20n - 4m = 4(n - m)$. 2. **Recall the distributive property:** $a(b - c) = ab - ac$. 3. **Apply the distributive property to the right side:** $$4(n - m) = 4 \cdot n - 4 \cdot m = 4n - 4m$$ 4. **Rewrite the equation:** $$20n - 4m = 4n - 4m$$ 5. **Subtract $4n$ from both sides:** $$20n - 4m - 4n = 4n - 4m - 4n$$ 6. **Simplify both sides:** $$\cancel{20n} - 4n - 4m = \cancel{4n} - 4m$$ $$16n - 4m = -4m$$ 7. **Add $4m$ to both sides:** $$16n - 4m + 4m = -4m + 4m$$ 8. **Simplify:** $$16n = 0$$ 9. **Divide both sides by 16:** $$\frac{16n}{\cancel{16}} = \frac{0}{\cancel{16}}$$ $$n = 0$$ **Final answer:** $n = 0$