Solve the equation 13 = (87 + 82)/x.

Step-by-step solution

1. 1

   Simplify the numerator before anything else.

   $87+82=169\quad\Longrightarrow\quad 13=\frac{169}{x}$

2. 2

   Clear the denominator. The unknown sits underneath, so multiply both sides by $x$. This is legitimate provided $x\neq 0$ — and $x=0$ was never allowed, since it would make the original fraction undefined:

   $13x=169$

3. 3

   Divide by $13$. Recognising $169=13^2$ makes the division immediate:

   $x=\frac{169}{13}=13$

4. 4

   Check and note why the answer equals the divisor. $\tfrac{169}{13}=13$ $\checkmark$. The coincidence is that $169$ is a perfect square, so the quotient equals the divisor; that is exactly the condition $x^2=169$ in disguise.