Why? Because if three hens laid three eggs in three days, then it means one hen lays one egg per day - so twelve hens, given twelve days, will lay twelve eggs.

Why? Because if three hens laid three eggs in three days, then it means one hen lays one egg per day - so twelve hens, given twelve days, will lay twelve eggs. That's the immediate reasoning, right? It's very easy to fall into that trap, and miss the fact that two of the variables were changed.