If 999x + 888y = 1332 and 888x + 999y = 555, then the value of x + y is?

Answer: Option B

$$\begin{array}{rl}& 999x+888y=1332\\ & \underline{888x+999y=555}\\ & 1887\left(x+y\right)=1887\\ & \Rightarrow x+y=\frac{1887}{1887}\\ & \Rightarrow x+y=1\end{array}$$