A coin purse has 3 OLD one-peso coins and 7 NEW one-peso coins. Suppose you randomly pick a coin and upon selecting, a dummy silver coin is placed on the purse as a placeholder for the coin you picked. If a second coin is picked from the coin purse, what is the probability that the second coin is an OLD one peso? *
7/25
14/50
17/50
3/50
+118673 P(old,old)= 3/10 * 2/10 = 6/100
P(new ,old) = 7/10*3/10 = 21/100
P(the second one is old ) = (6+21)/100 = 27/100
I appreciate it but that's not in the choices and I'm still stuck at getting an answer of 7/50 :(
+118673 hi, thanks for responding.
Please show hotw you got 7/50. If you do not show working I have no ability to discuss it with you.
I am sticking by my solution. I do not believe any of the given answers are correct.
If you are given a worked answers that gives a different answer from me, please present it here.
hi again, so I tried again and this is what I got
Assuming that an old coin was picked in the first, P(old, new) = P(2/10)(7/10)=7/50
Assuming that an either old or new was picked in the second and a dummy silver coin(maybe an old one or a new one)
P(old,new)= (1/10+1/10)(6/10+1/10) = 7/50
P=(7+7)/50=14/50
is this reasonable? if this is not I'll just try asking my classmates how they get it lol
this is giving me a headache and I still got 2 more problems to solve.
btw thanks for your kindness.
+118673 My interpretation of the question
A coin is chosen and not returned BUT a dummy coin is thrown into the mix so that there will still be 10 coins to chose from
Then a second coint is chosen. What is the probablility that this second coin is an old one.
I have answered this question correctly.
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Perhaps you are answering a different interpretation of the question ? I mean maybe you are trying to answering the intended question but it is not actually the question asked.
I think this maybe what you are trying to answer:
A coin purse has 3 OLD one-peso coins and 7 NEW one-peso coins. Suppose you randomly pick a coin and upon selecting, a dummy silver coin is placed on the purse as a placeholder for the coin you picked. If a second and third coin is picked from the coin purse, what is the probability that the second third coin is an OLD one peso?
Does that sound right to you? there are 3 picks so that is 2^3 = 8 outcomes.
I did look at this but I still got 27/100
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Unless you can think of a third interpretation of the question then I think none of those choices are correct.
Plus I do ot understand your working at all. 
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Perhaps you should move onto onother of you questions as I really believe that this one is a dud. 
+397 Drat, I typed something in and somehow it got lost.
Here's the gist of it.
It often helps to list all of the possible outcomes with their associated probabilities, their sum should equal 1.
If it does, you can be reasonably sure that you haven't missed anything.
So, we have
O, O = 3/10 * 2/10 = 6/100,
O, N = 3/10 * 7/10 = 21/100,
O, S = 3/10 * 1/10 = 3/100,
N, O = 7/10 * 3/10 = 21/100,
N, N = 7/10 * 6/10 = 42/100,
N, S = 7/10 * 1/10 = 7/100.
Total = 100/100, tick.
The probability that the second coin is an old Peso is 6/100 + 21/100 = 27/100.
If we disagree somewhere then at least we should be able to see where the disagreement is.
