b = u-2*a/2, b=20, a=100

b = u-2*a/2        b=20   a=100
einsetzen:          20 = u-2*100/2
vereinfachen:      20 = u-100     |+100
nach u auflösen: 120 = u

b = u - 2*a/2

u = b + 2*a/2

u = 20 + 2*100/2

u = 20 + 100 = 120

$${\mathtt{u}} = {\mathtt{20}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{2}}{\mathtt{\,\times\,}}{\mathtt{100}}}{{\mathtt{2}}}} \Rightarrow {\mathtt{u}} = {\mathtt{120}}$$

Gruß "radix"