Exercise 1 solution

Colony of 60,000 bees.

Two-thirds leave: 60,000 x 2/3 = 40,000 leave; 20,000 stay.
One-half leave:   60,000 x 1/2 = 30,000 leave; 30,000 stay.

Which to use: neither is certain. The article gives "approximately two-thirds" in one section and "about half" in another. A sensible plan allows for a range, 30,000 to 40,000 bees leaving (a loss of between one half and two thirds of the colony), and treats the real figure as unknown until checked against another source. Also note that not all 60,000 are workers, so the true numbers are slightly lower.