Only the middle one is taken: it is the highest of the lowest and the lowest of the highest. It is said that if there are twenty sheep and twenty goats, the middle one is taken. To determine this, one must evaluate the middle from the goats and the sheep, and a sheep is taken that is worth half the value of each of them. See: Ghinayat Dhawi Al-Ahkam 1: 178, and Al-Durr Al-Mukhtar 2: 22.