The food will last for 6 days for 20 animals.
If there were 10 more animals in the cattle, the total number of animals would be 20+10 = 30.
The food will last for the same 6 days but now it would be divided among 30 animals.
Therefore, the food would last for 6 days / 30 animals = 0.2 days per animal.
So, the food will last for 0.2 days if there were 10 more animals in the farmer's cattle.