Răspuns :
Notez numarul cu a
( a : 7 + 15 ) : 6 = 3
a : 7 + 15 = 3 × 6
a : 7 + 15 = 18
a : 7 = 18 - 15
a : 7 = 3
a = 3 × 7
a = 21
Proba
( 21 : 7 + 15 ) : 6 = ( 3 + 15) : 6 = 18 : 6 = 3
( a : 7 + 15 ) : 6 = 3
a : 7 + 15 = 3 × 6
a : 7 + 15 = 18
a : 7 = 18 - 15
a : 7 = 3
a = 3 × 7
a = 21
Proba
( 21 : 7 + 15 ) : 6 = ( 3 + 15) : 6 = 18 : 6 = 3