Answer:
Let:
x = code for country A
This is important: "the codes are three consecutive integers" That means,
x is code for country A
(x+1) is code for country B
(x+2) is code for country C
whose sum is 81" means
x + (x+1) + (x+2) = 90
3x+3=90
=3x=90-3
=3x=87
=x=87/3
=x=29
So, the codes are=29,30,31
To check 29+30+31=90