'''V<sub>rück</sub> = ( r(D,30) x A<sub>ges</sub> – ( r(D,2) x C<sub>Dach</sub> + r(D,2) x A<sub>FaG</sub> x C<sub>FaG</sub>)) x D x 60 / (10000 x 1000)'''
Vrück = ( r(D,30) x Ages – ( r(D,2) x CDach + r(D,2) x AFaG x CFaG)) x D x 60 / (10000 x 1000)