Exercise 2 solution

Timeline from the article's table:
Days 1-10:  cleaning the hive and feeding larvae
Days 11-15: "after this" the article says they begin building comb cells (no exact day range is given)
Days 16-20: receiving nectar and pollen from older workers and storing it
Day 21 onward: leaves the hive and forages for the rest of life ("after the 20th day")

Changes happen after day 10, at day 16, and after day 20.

Day 12: most likely building comb cells. What the article does not say: the exact days for comb building. The span 11 to 15 is my inference from the gap between "first 10 days" and "days 16 through 20"; the article only says "after this".