The key to solving Newton's problem is to ask how many cows can eat old grass and how many cows can eat new grass.

Therefore, we can think according to the following ideas:

① According to "10 cows can eat for 20 days", it can be calculated that 10×20=200 cows can eat 1 day.

② According to "15 cows can eat 10 day", it can be calculated that 15× 10= 150 cows can eat 1 day. This is because the grass on the grassland is 10 days (20 days)

(3) 1 day, how many cows can eat the original grass (excluding the newly grown grass)?

(10-5) × 20 = 5× 20 =100 (head)

Or: (15-5) ×10 =10 ×10 =100 (head)

Now there are 25 cows coming in, because the new grass on the grass is enough to raise 5 cows. Just calculate how many days the remaining 20 cows have eaten the original grass, and the result will come out.

100 ÷ (25-5) =100 ÷ 20 = 5 (days)

Only in this way can we get the answer step by step.

(1) How many cows can eat the new grass growing on the pasture every day;

( 10×20- 15× 10)÷(20- 10)

=(200- 150)÷ 10

=50÷ 10

=5 (head)

(2) 1 day How many cows have enough grass to eat?

(10-5) × 20 = 5× 20 =100 (head)

(3) How many days will it take to feed 25 cows with old grass and new grass on the pasture?

100 ÷ (25-5) =100 ÷ 20 = 5 (days)