a)
ab - a - b = 27
10 × a + b - a - b = 27
9 × a + b - b = 27
9 × a = 27
a = 27 : 9
a = 3 si b = 0
ab = 30
___________
30 - 3 - 0 = 27
b)
ab + a - b = 55
10 × a + b + a - b = 55
11 × a + b - b = 55
11 × a = 55
a = 55 : 11
a = 5 si b = 0
ab = 50
____________
50 + 5 - 0 = 55
