A roll of two 6s (the highest roll possible) on a pair of 6-sided dice. The probability of rolling boxcars in a single roll of two dice is 1/36, or 2.777...%.
In order to have a 50% chance of obtaining at least one boxcars in 
rolls of two dice, it must be true that
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(1)
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so solving for 
gives
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(2)
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In fact, rolling two dice 25 times gives a probability of
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(3)
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that at least once boxcars will occur.
