Of 50 students, 30 play soccer, 25 play basketball, and 10 play both sports. How many students play neither sport?

Use the inclusion-exclusion idea. Students who play at least one sport number $30+25-10=45$, where the $10$ who play both are subtracted so they are not counted twice. The students who play neither number $50-45=5$, choice A. Choice D ($45$) is the number who play at least one sport, not neither. Choice B ($10$) is the number who play both. Choice C ($15$) is $30-15$ or a similar slip. A Venn diagram helps: $20$ play only soccer, $15$ play only basketball, $10$ play both, and $5$ play neither, which totals $50$.