If you need urgent support, call 999 or go to your nearest A&E. To contact our Crisis Messenger (open 24/7) text THEMIX to 85258.
Options
Probability
Former Member
Posts: 1,876,323 The Mix Honorary Guru
can someone help me with this question please...
Urn 1 has 2 white balls and 4 red balls.
Urn 2 has 2 white balls and 1 red ball.
a ball is then randomly picked from urn 1 and placed into urn 2
what is the probability of picking a white ball from urn 2?
i think ive done it wrong...
there is 0.33 prob that a white ball will be picked from urn 1
if the white ball then goes into urn 2, there is 0.75 prob that a white ball will be picked from urn 2
from this i then multiplied the 2 together 0.33*0.75 and got 0.2475, but this seems wrong because even if a red ball was moved into urn 2, there would be a 0.5 prob that a white was picked from it.
Thanks
Urn 1 has 2 white balls and 4 red balls.
Urn 2 has 2 white balls and 1 red ball.
a ball is then randomly picked from urn 1 and placed into urn 2
what is the probability of picking a white ball from urn 2?
i think ive done it wrong...
there is 0.33 prob that a white ball will be picked from urn 1
if the white ball then goes into urn 2, there is 0.75 prob that a white ball will be picked from urn 2
from this i then multiplied the 2 together 0.33*0.75 and got 0.2475, but this seems wrong because even if a red ball was moved into urn 2, there would be a 0.5 prob that a white was picked from it.
Thanks
Post edited by JustV on
0
Comments
If that makes sense.
let W be the probability of pulling a known white ball, R be the probability of picking a known red ball and M that of getting the mystery ball
you know that W+R+M=1
does that help any?
xsazx thanks for ur post but not really understanding that one, because we are only interested in a white ball being picked from urn1 so the probability of a red ball being picked is irrelevant is it not?
god i wish they would just give us mark schemes!
its not irrelevant, otherwise they wouldnt say 'a random ball is picked from urn 1'.
It might just be a badly worded question, but it's not(imo) referring to the probability of them BOTH being white, because a white ball could still be picked from urn 2 even if the random ball from urn 1 was red.
probability of getting RED -
is 1/4 (the one red that started in urn 2) + ( 1/4 (the mystery ball) x 4/6 (the chances of it being red that you picked) )
P(r) = 1/4 + 4/6 x1/4
= 1/4 + 1/6
= 3/12 + 2/12
= 5/12
now, I'll let you work out what the probability of WHITE is for yourself.