Probability

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

Reply 1

Dowel

8

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

The answer is 14.
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.

Reply 2

Jellyharold

OP

Original post by Dowel

The answer is 14.
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?

Reply 3

Wahrheit

18

Original post by Dowel

The answer is 14.
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

Reply 4

ghostwalker

17

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.