Need help with this question, my textbook has the solution but I don't know how to get it.
15 coins in a bag. Three 5 rand coins Five 2 rand coins Seven 1 rand coins
What is the prob that if I pull out 5 random coins, it equals 10 rand The solution is 0.1752
I put all possible options that would give 10 rand over 15 choose 5, but unless I missed one which I'm pretty sure I haven't, I got the wrong answer of 0.441
There are ${15\choose 5}=3003$ ways to pick $5$ coins.
The only ways for them to add up to $10$ rands are if you choose $5$ two-rand coins (one way) or $1$ five-rand coin, $1$ two-rand coin, and $3$ one-rand coins (${3\choose1}{5\choose1}{7\choose3}=525$ ways.) So the probability is $${526\over3003}=0.17515817515817517...$$