This problem could be easily solved systematically through the use of algebra. First, let's denote x to be the unknown, which is the number of chickens in the hen house. Each chicken hen produces 2 eggs each. So, that means 2 eggs/chicken. Now using dimensional analysis, we should cancel out the eggs so that we can have a final answer in terms of number of chickens. In order to do this, the dimensional analysis should be
x = eggs*(chicken/egg)
Cancelling out similar terms, the units in eggs should be cancelled out leaving chickens. Therefore, our final answer should be
x = 42 eggs * (1 chicken/2 eggs) = 21 chickens
Hence, the answer should be 42÷2. When I look at the choices, the closest answer would be 42÷12. I can conclude that there must be some typographical error wherein you typed 2 instead of 12. Nevertheless, the general solution requires you divide the number of total eggs by the number of eggs each chicken lays.
