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Consider the 20 points of a 4×5 grid. You randomly choose two points from the 20 points. What is the probability that the two points belong to a horizontal or vertical line?

 Feb 28, 2021
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if the two points can be the same point, then the answer is $\boxed{\frac{(4+4)\cdot20}{20\cdot20}=\frac25}$

 

if the two points are different, then the answer is $\boxed{\frac{(4+3)\cdot20}{20\cdot19}=\frac7{19}}$

 Feb 28, 2021

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