Solution
= Solution
The observed positive counts are $200(0.18)=36$ and $400(0.08)=32$. Thus the responder-only estimate is
$$
\boxed{\widehat p_R=\frac{36+32}{600}=\frac{68}{600}\simeq11.33\%.}
$$
This estimates the proportion among the responders; it does not yet address either kind of nonresponse.