Probability (1 Viewer)

[mchew92]

A box contains disk which are numbers 2,2,3,3,3,4,5. Daniel takes one disk at random from the box, then takes another without replacement.

what is the probability of the second disk being a 2 (ans is 11/42) i keep getting 12/42, i think i may be doing sth rong? please help and explain.

 

[Timothy.Siu]

A box contains disk which are numbers 2,2,3,3,3,4,5. Daniel takes one disk at random from the box, then takes another without replacement.

what is the probability of the second disk being a 2 (ans is 11/42) i keep getting 12/42, i think i may be doing sth rong? please help and explain.

2 cases, one if he takes the 2 first and one if he doesn't
first case: 2/7 x 1/6
2nd case: 5/7 x 2/6

so i get 12/42 too, maybe its right.

 

[Top Secret]

2 cases, one if he takes the 2 first and one if he doesn't
first case: 2/7 x 1/6
2nd case: 5/7 x 2/6

so i get 12/42 too, maybe its right.

+1

 

[mchew92]

Ok thx,
what about this one
A bag contains one red disk, 2 black and 1 white
Kathy selects one and keeps it hidden, then selects a second and keeps that also hidden

if kathy shows us one disk is black. What is the probability that the disk that is still hidden is black.

Ans is1/3
but i agn got something different
this worksheet is shit

 

[Timothy.Siu]

Ok thx,
what about this one
A bag contains one red disk, 2 black and 1 white
Kathy selects one and keeps it hidden, then selects a second and keeps that also hidden

if kathy shows us one disk is black. What is the probability that the disk that is still hidden is black.

Ans is1/3
but i agn got something different
this worksheet is shit

this means she has at least one black disk.
if one of them is black, then you only have 3 disks to choose from. one black one white one red. so 1/3. lol pretty dodgey way.

 

[mchew92]

but she could have shown u the 2nd black disk she took
so wt bout like RB ,WB, BR, BW, BB etc..
thx

 

[scardizzle]

i get 1/5
my working:

possible outcomes with B:
RB WB BB BR BW

each has a 1/6 probability of occuring therefore the probability of selecting BB is 1/5 if one must be B

 

[Timothy.Siu]

makes sense to me.