September 2011 Brain Teaser Solution

Q: A chicken farmer has figured out that a hen and a half can lay an egg and a half in a day and a half. How many hens does the farmer need to produce one dozen eggs in six days? A: 3 hens

If a hen and a half can lay an egg and a half in a day and a half, then  in three days (twice the amount of time), the same hen and a half can lay three eggs and in six days (doubling again), the same hen and a half can lay six eggs. We want twelve eggs in six days so we need to double the number of hens from 1.5 to 3. Another solution comes from Tim at Riverdale City Grill Bronx, NY:

You need 3 hens b/c each hen lays an egg every 36 hours. There are 144 hours in 6 days. That means each hen lays 4 eggs every 6 days. 3x4=12.