# Maths help

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#1
Math query,
My wallet contains two 50p coins and five 20p coins only.
I need 70p to buy an item.
If I pick two coins at random from my wallet, what is the probability that I have picked the exact
The answer is 10/21 if someone would be able to show me their worked solution to this I would be grateful, thanks!
0
4 weeks ago
#2
(Original post by Dhhdndjdjjdjd)
Math query,
My wallet contains two 50p coins and five 20p coins only.
I need 70p to buy an item.
If I pick two coins at random from my wallet, what is the probability that I have picked the exact
The answer is 10/21 if someone would be able to show me their worked solution to this I would be grateful, thanks!
You get exactly 70p by picking 50p + 20p in either order.

Calculate these two probabilities and add them up.
0
#3
(Original post by RDKGames)
You get exactly 70p by picking 50p + 20p in either order.

Calculate these two probabilities and add them up.
Originally I got 2/7 * 5/7 which is 10/49. im still not quite sure how i would do this.
0
4 weeks ago
#4
(Original post by Dhhdndjdjjdjd)
Originally I got 2/7 * 5/7 which is 10/49. im still not quite sure how i would do this.
You are assuming that once a coin is picked it is put back ...

But it is not. There is no replacement, therefore the sample space reduces to 6 coins once the first one is picked, so the denominator on the second fraction would be 6.
0
#5
(Original post by RDKGames)
You are assuming that once a coin is picked it is put back ...

But it is not. There is no replacement, therefore the sample space reduces to 6 coins once the first one is picked, so the denominator on the second fraction would be 6.
Yes thank you for clearing that up but then my answer would be 5/21 and it is 10/21 so how would you get that as an answer?
0
4 weeks ago
#6
(Original post by Dhhdndjdjjdjd)
Yes thank you for clearing that up but then my answer would be 5/21 and it is 10/21 so how would you get that as an answer?
Order. Add the probability of picking 50p, then 20p to that of picking 20p, then 50p.
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