What is the probability that it was a penny?
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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?
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%.
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