There is no such ans like 7.55mv.
I think...
Quantization Error is (ε Max)= 2.55v/(2^8)=10 mv
so total error = 10mv+2.55mv = 12.55 mv(Correct Ans)
As per ur concept," if Qunts. error contain in 2.55mv other Cumulative error " which is not possible.
10-2.55 =7.45 mv(not the Ans)