19
References:
Math4Math Games (2012). Math puzzles brain teaser. [Kindle version]. Retrieved from Amazon.com.
Pierce, Rod. (25 Jan 2015). "Math is Fun - Puzzles". Math Is Fun. Retrieved 16 June 2015 from http://www.mathsisfun.com/puzzles
The Grabarchuk Family (2011). 100 Puzzle Quizzes. [Kindle version]. Retrieved from Amazon.com.