Step 1: Identify the given percentage changes

Let the tax rate decrease by $a = -20\%$ and the consumption increase by $b = +15\%$.

$a = -20, \quad b = +15$

Step 2: Apply the successive percentage formula

Tax revenue is the product of tax rate and consumption. The net percentage change formula is given by $a + b + \frac{a \times b}{100}$.

$\text{Net Effect} = -20 + 15 + \frac{(-20) \times 15}{100}$

Step 3: Simplify and calculate the final value

Substitute the values: $-20 + 15 = -5$, and $\frac{-300}{100} = -3$. Adding them gives $-5 - 3 = -8\%$.

$\text{Net Effect} = -5 - 3 = -8\%$

Step 4: Conclusion

A negative sign indicates a decrease. Therefore, the tax revenue decreases by $8\%$.

$8\% \text{ decrease}$