MathDB

On Halloween 31 children walked into the principal's office asking for candy. They can be classified into three types: Some always lie; some always tell the truth; and some alternately lie and tell the truth. The alternaters arbitrarily choose their first response, either a lie or the truth, but each subsequent statement has the opposite truth value from its predecessor. The principal asked everyone the same three questions in this order.
"Are you a truth-teller?" The principal gave a piece of candy to each of the 22 children who answered yes.
"Are you an alternater?" The principal gave a piece of candy to each of the 15 children who answered yes.
"Are you a liar?" The principal gave a piece of candy to each of the 9 children who answered yes.
How many pieces of candy in all did the principal give to the children who always tell the truth?

<span class='latex-bold'>(A) </span>7\qquad<span class='latex-bold'>(B) </span>12\qquad<span class='latex-bold'>(C) </span>21\qquad<span class='latex-bold'>(D) </span>27\qquad<span class='latex-bold'>(E) </span>31

Problem Statement