Answer:
This is an indirectly proportional relation, because the food will last less if the number of animals increases.
What we can do is a rule of three, as we know that the food lasts 30 days for 100 animals, and we want to know how many days does it last for 120:
30 days -- 100 cows
x days -- 120 cows
As it's an inderect rule of three, x = = 25.
The food will last 25 days for them.