100 × 10 - ( 100 - 2 × X - 60 ) × 10 = 900
1 000 - ( 100 - 2 × X - 60 ) × 10 = 900
( 100 - 2 × X - 60 ) × 10 = 1 000 - 900
( 100 - 2 × X - 60 ) × 10 = 100
100 - 2 × X - 60 = 100 : 10
100 - 2 × X = 10 + 60
2 × X = 100 - 70
2 × X = 30
X = 30 : 2
X = 15 → numarul, aflat prin metoda mersului invers
Verific:
1 000 - ( 100 - 2 × 15 - 60 ) × 10 = 1 000 - 10 × 10 = 900