Because the weather is getting colder and colder, the grass on the pasture is decreasing at a constant speed every day. You can feed 40 cows 10 day and 30 cows 12 day. Ask 20 cows for several days.

Let the grass eaten by 20 cows every day be R, and the grass reduced on the grassland every day be T, then the equation is obtained:

40 cows eat 10 day: 2RX 10+ 10T= the total amount of grass.

30 cows eat 12 days: 1. 5RX 12+ 12T= total grass.

2RX 10+ 10T= 1 .5RX 12+ 12T

R(R (grass eaten by 20 cows every day) =T (grass reduced on the grassland every day)

In Jed Grass Factory, 20 cows can eat less grass every day.

If 40 cows eat it, it means that 40 cows have eaten it for two days, and there is less grass for one day.

Now let's assume that if the grass on the grass factory does not continue to decrease.

Then 40 cows can eat10+10/2 =15 days.

Then 20 cows can naturally eat 30 cows, but in fact, 20 cows will continue to reduce enough grass for one day, so we can know the answer: 30/2= 15.

There is enough grass on the grassland for 20 cows 15 days.