In a survey of 100 traders in Lagos market, 60 sell rice, 45 sell beans, and some sell both. If 15 traders sell neither rice nor beans, how many traders sell both rice and beans?

Since 15 sell neither, those selling at least one item = 100 − 15 = 85. By inclusion-exclusion: n(R ∪ B) = n(R) + n(B) − n(R ∩ B), so 85 = 60 + 45 − n(R ∩ B), giving n(R ∩ B) = 105 − 85 = 20. Options A, B, and D result from errors in applying the inclusion-exclusion formula.