Hmmm....I think I'm with the 2-1/4 days answer. Here's why:
1.5 chickens = 1.5 eggs in 1.5 days. Multiplying the number of chickens laying will multiply the number of eggs laid by the same factor, therefore:
15 chickens = 15 eggs in 1.5 days. Thusly, 10 chickens = 10 eggs in 1.5 days. So the chickens will need another half of 1.5 to accomplish the other five eggs, so....
10 chickens = 15 eggs in 2.25 days.
BTW, they don't lay 10 eggs the first day, they lay 10 eggs the first day and 1/2.