7 + 6 × ( 5 + 2 × a ) : 5 = 25
6 × ( 5 + 2 × a ) : 5 = 25 - 7
6 × ( 5 + 2 × a ) : 5 = 18
6 × ( 5 + 2 × a ) = 18 × 5
6 × ( 5 + 2 × a ) = 90
5 + 2 × a = 90 : 6
5 + 2 × a = 15
2 × a = 15 - 5
2 × a = 10
a = 10 : 2
a = 5 → numarul necunoscut, aflat prin metoda mersului invers