Charlie Piechart and the Case of the Missing Hat - Charlie Piechart

"Charlie Piechart's second math mystery takes place at school--when a purple, sparkly cone-shaped princess hat goes missing! Margot needs her hat in time for the school play (today!), so Charlie and his dog companion, Watson, are on the case--finding evidence of sparkles and purple paint and keeping on the lookout for geometric clues that might lead to the missing hat."--Amazon.com.