Dhhdndjdjjdjd
Badges: 9
Rep:
?
#1
Report Thread starter 4 weeks ago
#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
money to buy the item?
The answer is 10/21 if someone would be able to show me their worked solution to this I would be grateful, thanks!
0
reply
RDKGames
Badges: 20
Rep:
?
#2
Report 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
money to buy the item?
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
reply
Dhhdndjdjjdjd
Badges: 9
Rep:
?
#3
Report Thread starter 4 weeks ago
#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
reply
RDKGames
Badges: 20
Rep:
?
#4
Report 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
reply
Dhhdndjdjjdjd
Badges: 9
Rep:
?
#5
Report Thread starter 4 weeks ago
#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
reply
RogerOxon
Badges: 21
Rep:
?
#6
Report 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.
1
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Back
to top
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

Should there be a new university admissions system that ditches predicted grades?

No, I think predicted grades should still be used to make offers (622)
33.73%
Yes, I like the idea of applying to uni after I received my grades (PQA) (776)
42.08%
Yes, I like the idea of receiving offers only after I receive my grades (PQO) (364)
19.74%
I think there is a better option than the ones suggested (let us know in the thread!) (82)
4.45%

Watched Threads

View All
Latest
My Feed