15. \( 11 \sqrt{5}+7 \sqrt{5}= \)

Combining the terms, you have \( (11 + 7) \sqrt{5} = 18 \sqrt{5} \). So, the final answer is \( 18 \sqrt{5} \). To understand this better, think of \(\sqrt{5}\) as a "variable" or common factor that can simplify your calculations. Just like adding apples and oranges, you can only combine like terms, which in this case are both multiples of \(\sqrt{5}\), resulting in a delicious fruit basket of \( 18 \sqrt{5} \)!