How many ways can you choose ten tadpoles from a pool of 20?

What do you mean but if you mean what I think you mean then you take half of the tadpoles 

Bat wings and bird wings are analogous as flight structures: their structure and function have evolved by different routes from a flightless reptilian ancestor. Note for example that a good portion of the flight surface in bats consists of a membrane stretched between the extended digits (chiropatagium), whereas the digits of the bird are relatively small and do not support the flight surface.