As the weather gets colder and colder, the grass on the pasture is not growing, but decreasing at a fixed rate. It is known that grass on a certain field can feed 20 cows for 5 days.

Weeding days per day 1 cow: 20*5- 15*6= 10.

The number of days that the original grass can support 1 cow: 20 * 5+5 *10 =150 =15 * 6 *10.

3-day reduction 1 number of days for one cow to eat grass: 3* 10=30.

Number of cattle that can be released in 3 days: (150-30)/3=40.

So you can feed 40 cows for 3 days.

Suppose that the grass that each cow eats every day is X and the grass that the pasture reduces every day is Y, then according to the grass that can be used in the original pasture.

20*5x+5y= 15*6x+6y, y= 10x, so there is grass 150x.

So we can feed (150x-3y)/(3x)=40 cows for 3 days.