There are 8C2 = 28 ways to choose 2 dimes from the 8 dimes in Annie's purse. There are 21C2 = 210 ways to choose 2 coins from the 21 coins in Annie's purse.
Of the 210 ways to choose 2 coins, 28 of the choices will result in 2 dimes being chosen. The probability of choosing 2 dimes is 28/210 = 2/15.
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<em>Comment on nCk</em>
The number of ways to choose k objects from n, when order does not matter, is ...
Multiplying with 6 is like multiplying with 3 because, you just double it. Let's say 3 × 4 = 12 Its the same as 6 × 2 = 12 ... which is just 3 + 3 + 3 + 3 . ((:
