what does the u stand for?

what does the u stand for ? 5=u/2-10

$\begin{array}{rcll} 5 &=& \frac{u}{2}-10 \\ \frac{u}{2}-10&=& 5 \quad & | \quad + 10 \\ \frac{u}{2}-10+10&=& 5 + 10 \\ \frac{u}{2}+0&=& 15 \\ \frac{u}{2} &=& 15 \quad & | \quad \cdot 2 \\ u &=& 15 \cdot 2 \\ \mathbf{u} &\mathbf{=}& \mathbf{30} \\ \end{array}$