1. **State the problem:** Subtract 20 from the expression $\frac{15b}{23}$. 2. **Write the expression:** $\frac{15b}{23} - 20$. 3. **Find a common denominator:** The denominator is 23, so rewrite 20 as $\frac{20 \times 23}{23} = \frac{460}{23}$. 4. **Rewrite the subtraction:** $$\frac{15b}{23} - \frac{460}{23}$$ 5. **Combine the fractions:** $$\frac{15b - 460}{23}$$ 6. **Final answer:** The expression after subtracting 20 from $\frac{15b}{23}$ is $$\frac{15b - 460}{23}$$.