Because 7 students said they didn't like either rock or country, that leaves 93 students (100 – 7) who voted for rock, country, or both. If each of the remaining 93 students had been able to vote for either country or rock, but not both
, there would have been only 93 votes, but there were 147 more votes cast (90 + 57). The remaining 54 votes (147 – 93) must have been cast as second
votes. Therefore, 54 students must have voted twice — once for rock, and once for country.