What is the probability that it was a penny?


Mary had a coin purse with fifty coins, totaling exactly $1.00.  Unfortunately, while counting her change, she dropped one coin.  What is the probability that it was a penny?

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    There is an 85% probability that Mary dropped a penny.  There are two (and only two) combinations of 50 coins that will add up to $1.00.  These are:

    40 Pennies, 2 Dimes, 8 Nickels, and
    45 Pennies, 2 Dimes, 2 Nickels, 1 Quarter

    With the first scenario alone, there would be a 80% probability, and the second scenario alone equates a 90% probability, respectfully.  But because we don’t know which she had, the probability is the average of the two, or 85%.

    pnikam Expert Answered on 22nd August 2015.
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