Here's my best shot at this one....
Assuming 30 days in a month, there are 30 days//2.5 days = 12 producing cycles in the month
And in each of these cycles......1/2 of an egg is produced....so.....12 * 1/2 = 6 eggs are produced in a month by 1 1/2 chickens
Then......1 chicken must be able to produce only 2/3 of what 1.5 chickens can produce in a month since 1/1.5 = 2/3.....so......(2/3) * 6 = 4 eggs
