In a survey, 50% of respondents said they like coffee, 40% said they like tea, and 20% said they like both. What percent of respondents…

Use the inclusion-exclusion formula: $P(\text{coffee or tea}) = P(\text{coffee}) + P(\text{tea}) - P(\text{both})$. Substitute: $50\% + 40\% - 20\% = 70\%$, choice A. People who like both are counted in each of the first two percentages, so their share must be subtracted once. Choice B (90%) adds 50% and 40% without correcting for the overlap. Choice C (30%) is the share who like coffee only (50% minus 20%), and choice D (20%) is the share who like both. A Venn diagram makes this clear: 30% coffee only, 20% both, 20% tea only, total 70%.