Răspuns :
Notez numarul cu a
a × 2 : 4 = 5
a × 2 = 5 × 4
a × 2 = 20
a = 20 : 2
a = 10 → numarul aflat prin metoda mersului invers
Verific: 10 × 2 : 4 = 20 : 4 = 5
a × 2 : 4 = 5
a × 2 = 5 × 4
a ×2 = 20
a = 20 : 2
a = 10
10 × 2 : 4 = 5
a × 2 = 5 × 4
a ×2 = 20
a = 20 : 2
a = 10
10 × 2 : 4 = 5