Of the 20 people who each purchased 2 tickets to a concert, some used both tickets, some used only 1 ticket, and some us

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Of the 20 people who each purchased 2 tickets to a concert, some used both tickets, some used only 1 ticket, and some used neither ticket. What percent of the tickets that were purchased by the 20 people were used by those people?

(1) Of the 20 people, 10 used only 1 ticket.

(2) Of the 20 people, 4 used neither ticket.

Answer: C

Source: GMAT Prep