Another Bio question

[Comet208]

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I found this on TopScore and got me very confused:

Mom and dad want only 3 boys; what are their chances??

since the order isnt important here i thought, they could get
3B
3G
2B1G
2G1B
therefore making the chance 1:4. but their answer is 1:8 which they got it from 2^3 = 8.

dont we use the 2^n in probability when order is important??? what is the proper approach for this??

 

[Rezdawg]

This is how I approach it...

For their first child, there is a 1/2 chance that it is a boy.
For their second child, there is a 1/2 chance that it is a boy.
For their third child, there is a 1/2 chance that it is a boy.

Therefore, for them to ONLY have 3 boys, the chances are (1/2)(1/2)(1/2) = 1/8

 

[Rezdawg]

Comet, the way you approached it could be right as well, you just didnt follow through fully.

They could have...

BBB
BBG
BGB
GBB
GGB
GBG
BGG
GGG

Therefore, there is a 1 in 8 chance that it is BBB.

 

[Comet208]

thx man, your (1/2)^2 makes a lot of sense.