7 581 + (100 + a : 2) x 6 - 5 360 = 3 001
7 581 + (100 + a : 2) x 6 = 3 001 + 5 360
7 581 + (100 + a : 2) x 6 = 8 361
(100 + a : 2) x 6 = 8 361 - 7 581
(100 + a : 2) x 6 = 780
100 + a : 2 = 780 : 6
100 + a : 2 = 130
a : 2 = 130 - 100
a : 2 = 30
a = 30 x 2
a = 60 → numar aflat prin metoda mersului invers