It takes Harland 15 minutes to rake all leaves in the front yard. If Trudy scatters the leaves while Harland rakes, it takes him 20 minutes to rake the leaves. How many minutes does it take Trudy to scatter all the leaves?

This can be treated as a work problem and we can solve it as follows:

Harland working alone = 15 minutes

Trudy scatters the leaves + Harland rakes = 20 minutes

We set scattering as a negative value.

1/15 - 1/x = 1/20

x = 60 minutes for Trudy to scatter the leaves alone