### Instruction: If $\sqrt{400}=\sqrt{81}+\sqrt{n}$, then what is the value of $n$?### Response: I know that $\sqrt{400}=20$ and $\sqrt{81}=9$, so I can rewrite the equation as $20=9+\sqrt{n}$.
To isolate $\sqrt{n}$, I can subtract $9$ from both sides and get $11=\sqrt{n}$.
Now I need to undo the square root by squaring both sides, and I get $121=n$.
I can check my answer by plugging it back into the original equation and see if it works.
$\sqrt{400}=\sqrt{81}+\sqrt{121}$, and indeed, $20=9+11$, so I'm confident that $n=121$.