Three fourths of the offspring of two fruit flies have short wings, while the rest have wild-type wings . In a batch of offspring from those two flies, 100 have wild-type wings. How many of the flies have short wings?

If three-fourths of the offspring of two fruit flies have short wings and the rest have wild-type wings, then [tex]1-\frac{3}{4}=\frac{1}{4}[/tex] of the offspring must have wild-type wings. This means that the number of offspring that have short wings must be [tex]\frac{3/4}{1/4}=\frac{3}{4}\cdot \frac{4}{1}=3[/tex] times larger than the number of offspring that have wild-type wings. Since there are 100 offspring who have wild-type wings, there must be [tex]100\cdot 3=\boxed{300}[/tex] that have short wings.

Alternative:

If three-fourths of the offspring of two fruit flies have short wings and the rest have wild-type wings, then [tex]1-\frac{3}{4}=\frac{1}{4}[/tex] of the offspring must have wild-type wings. Therefore, 100 represents one-fourth of the total number of offspring, meaning there must be [tex]100\cdot 4=400[/tex] offspring total. Thus, there are [tex]400-100=\boxed{300}[/tex] offspring with short wings.