The number of sheep is:

500×3= 1500 heads

The total number of cattle and sheep is:

1500+500=2000 heads

Suppose the number of cows is changed to sheep, and then calculate:

Because "20 cows or 30 sheep"

So every 20 cows equals 30 sheep,

500/20× 30 = 750 heads

(750+ 1500)÷30=75

A: There are 2,000 cattle and sheep, and the farm needs 75 breeders.