This question is trickier than it looks! It stumped the test-makers.
Playable demonstration: Steve Phelps has made a Geogebra worksheet that you can try it out for yourself
http://tube.geogebra.org/student/m1442749
Blog post: http://mindyourdecisions.com/blog/2015/07/19/this-psat-question-stumped-almost-everyone-attaching-a-tetrahedron-to-a-pyramid-sunday-puzzle/
References:
Pyramid Power: Searching for an Error in Test Scoring with 830,000 Helpers. Howard Wainera. The American Statistician. Volume 37, Issue 1, 1983.
http://onlinelibrary.wiley.com/doi/10.1002/j.2333-8504.1981.tb01262.x/abstract
New York Times. “Youth Outwits Merit Exam, Raising 240,000 Scores.” Edward B. Fiske. March 17, 1981.
http://www.nytimes.com/1981/03/17/us/youth-outwits-merit-exam-raising-240000-scores.html
In researching this video, I found this problem was featured in the New York Times Worldplay puzzle a couple years ago: http://wordplay.blogs.nytimes.com/2013/09/23/pyramid-2/
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My Books
“The Joy of Game Theory” shows how you can use math to out-think your competition. (rated 4/5 stars on 23 reviews) https://www.amazon.com/gp/product/1500497444
“The Irrationality Illusion: How To Make Smart Decisions And Overcome Bias” is a handbook that explains the many ways we are biased about decision-making and offers techniques to make smart decisions. (rated 5/5 stars on 1 review) https://www.amazon.com/gp/product/1523231467/
“Math Puzzles Volume 1” features classic brain teasers and riddles with complete solutions for problems in counting, geometry, probability, and game theory. Volume 1 is rated 4.5/5 stars on 11 reviews. https://www.amazon.com/gp/product/1517421624/
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