1. **State the problem:** Simplify the expression $5a \times a$. 2. **Recall the rule:** When multiplying powers with the same base, add the exponents: $a^m \times a^n = a^{m+n}$. 3. **Apply the rule:** Here, $5a \times a = 5 \times a^1 \times a^1 = 5 \times a^{1+1} = 5a^2$. 4. **Final answer:** The simplified expression is $5a^2$.