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Help on Math Challenge I-B(c) (AMC8/MathCounts) Winter 2017-2018 (Counting & Probability)

 
 
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Re: Help on Math Challenge I-B(c) (AMC8/MathCounts) Winter 2017-2018 (Counting & Probability)
by David Reynoso - Tuesday, April 17, 2018, 4:23 PM
 

Since you do not know what $m$ and $n$ are, your answer should be a "formula" that would work for any two $m$ and $n$ that satisfy the conditions of the problem.