x total suprafata, 5x/12 egal 100, 5x egal 1200, x egal 240, y cat a arat in a treia zi, unu intreg 1/2 egal 3/2, 100 plus 3y/2 plus y egal 240, y plus 3y/2 egal 140; 5y/2 egal 140, 5y egal 280, y egal 56, 3 ori 56/2 egal 84, proba 100 plus84 plus 56 egal 240