Răspuns :
[ 58 - 2 × ( 1 + 27 : d ) ] : 5 = 10
58 - 2 × ( 1 + 27 : d ) = 10 × 5
2 × ( 1 + 27 : d ) = 58 - 50
2 × ( 1 + 27 : d ) = 8
1 + 27 : d = 8 : 2
1 + 27 : d = 4
27 : d = 4 - 1
27 : d = 3
d = 27 : 3
d = 9 → numarul, aflat prin metoda mersului invers