Question 609372
12 goat = 1 field per day (fpd)
15 cow = 1 fpd
20 horse = 1 fpd

1 goat eats at the rate of 1/12 fpd
1 cow eats at the rate of 1/15 fpd
1 horse eats at rate of 1/20 fpd

rate of consumption = 1/12 + 1/15 + 1/20 fpd 

thats 5/60 + 4/60 + 3/60 = 12/60 fpd

= 1/5 of a field a day.

I make it 5 days