Set the grass amount of grassland A to 1 and the grass amount of grassland A every day, and every cow will eat B every day.

Then the equation can be listed according to the conditions, 36 (3b-a) =120 (5b-a) =1.

The solution is b = 1/90 and a =1180.

So eight cows consume112 every day.

So it takes 12 days.

If you have any questions, just ask.

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