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
Blog post: http://mindyourdecisions.com/blog/2015/07/19/this-psat-question-stumped-almost-everyone-attaching-a-tetrahedron-to-a-pyramid-sunday-puzzle/
Pyramid Power: Searching for an Error in Test Scoring with 830,000 Helpers. Howard Wainera. The American Statistician. Volume 37, Issue 1, 1983.
New York Times. “Youth Outwits Merit Exam, Raising 240,000 Scores.” Edward B. Fiske. March 17, 1981.
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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