[ 2 × ( 2 × a + 2 × 2 ) + 4 ] : 15 = 4
2 × ( 2 × a + 4 ) + 4 = 4 × 15
2 × ( 2 × a + 4 ) + 4 = 60
2 × ( 2 × a + 4 ) = 60 - 4
2 × ( 2 × a + 4 ) = 56
2 × a + 4 = 56 : 2
2 × a + 4 = 28
2 × a = 28 - 4
2 × a = 24
a = 24 : 2
a = 12 → numarul, aflat prin metoda mersului invers
