1. **State the problem:** Simplify the expression $$3\sqrt{10} + 10\sqrt{10}$$. 2. **Formula and rules:** When adding or subtracting radical expressions, you can combine them only if they have the same radicand (the number inside the square root). Here, both terms have $$\sqrt{10}$$, so they are like terms. 3. **Combine like terms:** $$3\sqrt{10} + 10\sqrt{10} = (3 + 10)\sqrt{10}$$ 4. **Simplify the coefficients:** $$= 13\sqrt{10}$$ 5. **Final answer:** The expression in simplest radical form is $$13\sqrt{10}$$.