The blue driving gear is a teeth-uncompleted one.
Its number of remained teeth Z1c = 1
Its number of teeth (teeth-completed) Z1 = 40
Its number of cut-off teeth Z1f = Z1 – Z1c = 39.
The number of the green driven gear (teeth-completed) Z2 = 20
When the blue makes 1 revolution, the green makes 3/20 revolution.

Avoid wrong calculation:
When the blue makes 1 revolution, the green makes Z1c/Z2 revolution.
For this case it means when the blue makes 1 revolution, the green makes 1/20 revolution (!).