How to do problem no. 90 from Math Counts School Handbook 2011 2012

Team: 
The Lifes
Topic: Algebraic Expressions & Equations
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This video shows how to do problem no. 90 from Math Counts School Handbook 2011 2012

Common Core Standards: 6.EE.3
Problem #: 
90 (2011-2012)
Problem Difficulty Level: 5 [?]