Răspuns:
[tex]\boldsymbol{ \red{40}}[/tex]
Metoda mersului invers:
9000 - (6400 - Y:5:2) = 2604
6400 - Y:5:2 = 9000 - 2604
6400 - Y:5:2 = 6396
Y:5:2 = 6400 - 6396
Y:5:2 = 4
Y:5 = 4·2
Y:5 = 8
Y = 8·5
Y = 40
Verificare: 9000 - (6400 - 40:5:2) = 9000 - (6400 - 8:2) = 9000 - (6400 - 4) = 9000 - 6396 = 2604 (corect!)