Răspuns :
a +b = 90
a : b = 5
a = 5b
5b + b = 90
6b = 90
b = 90 : 6
b = 15
a = 15 x 5 = 75
75 + 15 = 90
75 : 15 = 5
a : b = 5
a = 5b
5b + b = 90
6b = 90
b = 90 : 6
b = 15
a = 15 x 5 = 75
75 + 15 = 90
75 : 15 = 5