Răspuns :
57 x 7= 399, operatia inversa: 399: 7= 57
90: 10 = 9, operatia inversa: 9 x 10= 90
72: 8 =9, operatia inversa: 9 x 8= 72
90: 10 = 9, operatia inversa: 9 x 10= 90
72: 8 =9, operatia inversa: 9 x 8= 72