A very interesting two minute puzzle from Martin Gardner’s The Colossal Book of Short Puzzles and Problems. Please do share this with your children as well.
If you happen to meet two of the Jones sisters (this assumes that the two are random selections from the set of all the Jones sisters), it is an exactly even-money (that is 50-50) bet that both the girls will be blue-eyed. What is your best guess as to the total number of blue-eyed Jones sisters?
As always, please send your answers as comments within the blog (preferred), or send an e-mail to alokgoyal_2001@yahoo.com. Please do share the puzzle with others if you like, and please also send puzzles that you have come across that you think I can share in this blog.
Happy Sunday!
Alok Goyal's Puzzles Page 
3 blue eyed girls because of 3C2/4C2 is 50:50
Let the number of sisters be N and number of blue eyed be B.
BC2 / NC2 = 0.5
B(B-1)*2 = N*(N-1)
One of N or N-1 is a multiple of 4
N=4,5,8,9,12,13…
N=4, B=3 fits the bill on first try š