All about flooble | fun stuff | Get a free chatterbox | Free JavaScript | Avatars    
perplexus dot info

Home > Probability
The Last Baby (Posted on 2003-10-08) Difficulty: 4 of 5
A baby is added to a hospital nursery. Before the baby was added there were two boys in the nursery and an uncounted number of girls. After the new baby is added a baby is selected at random among all the babies. The selected baby is a boy.

What is the probability that the added baby was a girl?

See The Solution Submitted by Ravi Raja    
Rating: 2.7778 (9 votes)

Comments: ( Back to comment list | You must be logged in to post comments.)
re: assumption | Comment 17 of 23 |
(In reply to assumption by A)

No, no assumption was made that the boy was picked out of the group following the majority. Remembering that this is independent of the number of girls originally present, let's just take the example of the case where 4 girl babies were originally present. The following table shows on each line the original 2 boys and 4 girls followed by either boy or girl (in equal number, representing the possible newborn), and the 7 possible choices of baby to select, and whether that particular baby is a boy or a girl.

You can see that even when girls are in the majority as in this instance, out of the five times that a boy was chosen under these circumstances, two of the times it was after a girl had been born and three after a boy had been born, although in no instance would a boy have been in the majority.

boy boy girl girl girl girl; girl & chose baby 1 which is a boy
boy boy girl girl girl girl; girl & chose baby 2 which is a boy
boy boy girl girl girl girl; girl & chose baby 3 which is a girl
boy boy girl girl girl girl; girl & chose baby 4 which is a girl
boy boy girl girl girl girl; girl & chose baby 5 which is a girl
boy boy girl girl girl girl; girl & chose baby 6 which is a girl
boy boy girl girl girl girl; girl & chose baby 7 which is a girl
boy boy girl girl girl girl; boy & chose baby 1 which is a boy
boy boy girl girl girl girl; boy & chose baby 2 which is a boy
boy boy girl girl girl girl; boy & chose baby 3 which is a girl
boy boy girl girl girl girl; boy & chose baby 4 which is a girl
boy boy girl girl girl girl; boy & chose baby 5 which is a girl
boy boy girl girl girl girl; boy & chose baby 6 which is a girl
boy boy girl girl girl girl; boy & chose baby 7 which is a boy


  Posted by Charlie on 2003-10-11 23:01:58

Please log in:
Login:
Password:
Remember me:
Sign up! | Forgot password


Search:
Search body:
Forums (0)
Newest Problems
Random Problem
FAQ | About This Site
Site Statistics
New Comments (0)
Unsolved Problems
Top Rated Problems
This month's top
Most Commented On

Chatterbox:
Copyright © 2002 - 2017 by Animus Pactum Consulting. All rights reserved. Privacy Information