1. **State the problem:** Simplify the expression $2^x + 2^{x-1} + 2^{x-2} + 2^{x-3}$. 2. **Recall the properties of exponents:** When adding terms with the same base but different exponents, factor out the common term with the smallest exponent or the largest exponent for easier simplification. 3. **Factor out the smallest power:** The smallest exponent here is $x-3$, so factor out $2^{x-3}$: $$2^x + 2^{x-1} + 2^{x-2} + 2^{x-3} = 2^{x-3}(2^3 + 2^2 + 2^1 + 1)$$ 4. **Calculate the terms inside the parentheses:** $$2^3 = 8, \quad 2^2 = 4, \quad 2^1 = 2, \quad 1 = 1$$ So, $$8 + 4 + 2 + 1 = 15$$ 5. **Write the simplified expression:** $$2^{x-3} \times 15 = 15 \times 2^{x-3}$$ **Final answer:** $$15 \times 2^{x-3}$$