Exercise 1 solution

1 kg of nectar at 80 percent water: 0.8 kg water, 0.2 kg solids.
Honey at 20 percent water: the 0.2 kg of solids is 80 percent of the honey, so honey mass = 0.2 / 0.8 = 0.25 kg.
So 1 kg of nectar gives about 0.25 kg of honey, i.e. about 4 kg of nectar per 1 kg of honey.

Approximate because: the article says "nearly 80 percent" and "less than 20 percent", not exact figures, and it ignores any other losses (for example sugar used by the bees themselves). Real nectar varies.