Răspuns :
( 18 + B) : 3 = 30 → folosesc metoda mersului invers
18 + B = 30 × 3
18 + B = 90
B = 90 - 18
B = 72
Verific: ( 18 + 72 ) : 3 = 90 : 3 = 30
18 + B = 30 × 3
18 + B = 90
B = 90 - 18
B = 72
Verific: ( 18 + 72 ) : 3 = 90 : 3 = 30