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# Probability watch

1. So I really don't understand this question could someone please explain and show working out answers ect
.

Bill has some counters in a bag
3 of the counters are red
7 of the counters are blue
And the rest of the counters are yellow

Bill takes a counter from the bag

The probability he takes a yellow is 2/7
How many counters are in the bag before Bill takes a counter
2. (Original post by Jellyharold)
So I really don't understand this question could someone please explain and show working out answers ect
.

Bill has some counters in a bag
3 of the counters are red
7 of the counters are blue
And the rest of the counters are yellow

Bill takes a counter from the bag

The probability he takes a yellow is 2/7
How many counters are in the bag before Bill takes a counter
call the number of yellow counters in the bag x
so the overall number of counters is (10+x)
we know that x makes up 2/7 of (10+x)
we know that 10 makes up 5/7 of (10+x)
to get 1/7 we divide 10 by 5 and we get 2.
therefore 2 is 1/7 of the total number of counters in the bag.
times this number by 7 to get 14.
3. (Original post by Dowel)
call the number of yellow counters in the bag x
so the overall number of counters is (10+x)
we know that x makes up 2/7 of (10+x)
we know that 10 makes up 5/7 of (10+x)
to get 1/7 we divide 10 by 5 and we get 2.
therefore 2 is 1/7 of the total number of counters in the bag.
times this number by 7 to get 14.
What would the answer be if you wanted to work out the amount of yellow in the bag?
4. (Original post by Dowel)
call the number of yellow counters in the bag x
so the overall number of counters is (10+x)
we know that x makes up 2/7 of (10+x)
we know that 10 makes up 5/7 of (10+x)
to get 1/7 we divide 10 by 5 and we get 2.
therefore 2 is 1/7 of the total number of counters in the bag.
times this number by 7 to get 14.
Not a criticism, but wouldn't it have been so much nicer to call the number of yellow "y" instead of x? Haha
5. (Original post by Jellyharold)
What would the answer be if you wanted to work out the amount of yellow in the bag?
if there are 14 in the bag, and 3 are red, 7 are blue, then 14-3-7=4 are yellow.

Alternatively, from scratch, if x are yellow, then there are 10+x counters.

Probability of choosing yellow is 2/7 = x/(10+x) and solve for x.
6. (Original post by Jellyharold)
What would the answer be if you wanted to work out the amount of yellow in the bag?
yellow makes up 2/7 of the overall number of counters so you do the value of 1/7 (2) x 2 to get 4.

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Updated: June 2, 2016
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