Input: the yellow gear.
Its number of remained teeth Z1c = 9
Its number of teeth (teeth-completed) Z1 = 20
Its number of cut-off teeth Z1f = Z1 – Z1c = 11
Output: the green gear.
Its number of teeth (teeth-completed) Z2 = 20.
When the yellow makes 1 revolution, the green makes 1/2 revolution.