10 + 10 : { [ 10 + 10 × (X - 1 ) : 10 - 10 ]} = 11
10 : [ 10 + 10 × (X - 1 ) : 10 - 10 ] = 11 - 10
10 : [ 10 + 10 × (X - 1 ) : 10 - 10 ] = 1
10 + 10 × (X - 1 ) : 10 - 10 = 10 : 1
10 + 10 × (X - 1 ) : 10 - 10 = 10
10 + 10 × (X - 1 ) : 10 = 10 + 10
10 + 10 × (X - 1 ) : 10 = 20
10 × (X - 1 ) : 10 = 20 - 10
10 × (X - 1 ) = 10 × 10
10 × (X - 1 ) = 100
X - 1 = 100 : 10
X = 10 + 1
X = 11 → numarul, aflat prin metoda mersului invers