Mary had exactly 21 coins in her purse, total value of a round dollar.
While counting her money, she dropped a coin.
Evaluate the probability it was a penny.
From Charlie's post it appears we need to have some information about the distribution of the coins Mary has. The most recent data I could find is from https://en.wikipedia.org/wiki/United_States_Mint_coin_production
The most recent entry in the chart is distribution for 2015:
Charlie's fourth set (0 half-dollar, 2 quarters, 3 dimes, 1 nickel, 15 pennies) is closest to the 2015 Mint distribution. So if the Mint distribution is taken as our basis of coin distribution then the probability Mary dropped a penny is likely to be close to 15/21.