OK, I'll play. Two thirds of the population is sterile. Since it is not stated that this condition is sex-selected we can assume that this also means that two thirds of the population of each sex is sterile. If only one couple is to marry, and the population is infinite, the likelihood that each of the couple is fertile is 1/3 and the likelihood that the couple itself is fertile is therefore 1/9.
Or in other words for every three males and three females there is on the average one fertile pair, four pairings in which one member is fertile and four pairings in which neither member is fertile. The likelihood of a fertile pair at random is 1/9.
If everybody marries then for every fertile pair there are two infertile pairs actually formed on the average, and the percentage of fertile pairs then increases to 1/3.
I don't know what you find preferable in "birth (v.)" as opposed to "have". As for marry" I intended that as a simple euphemism.
There is one thing in this realm that bothers me, though - it's the silly phrase "They're pregnant". Mrs. A has done that, but I certainly never did. So "they" never seemed to be giving due credit.
There are four hens-and-a-half in six hens, therefore six hens will lay 4 x one egg-and-a-half, or six eggs, in one day and a half. Since there are four and two-thirds one-days-and-a-half in a week, you will get 6 eggs/dayandahalf x 4.67 day-and-a-halfs/week = 28 eggs per week.
Curiously, this is also the same number of pancakes it takes to shingle a doghouse, south of the Mason-Dixon line.