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Three coins are randomly placed into different positions on a 4×10 grid. The probability that no two coins are in the same row or column can be expressed as a/b where a and b are coprime positive integers. What is the sum of a+b?
GEOmetry(Special Coin Placement) -
The total number of ways of choosing rows is 4C3, columns 10C3
Total number of ways of choosing 3 tiles among 40(4X10) is 40C3
Answer = (4C3*10C3)/40C3