Suppose: the elder brother's age is A, the elder brother's age is B, and the younger brother's age is A-B.

1) My brother said, "When I was your age, you were only 5 years old."

So my brother's age five years ago is equal to my brother's age now.

Then a-b-5=b

2) The younger brother said, "When I reach your age this year, you will be 17 years old."

Then the age of the younger brother plus the age of the older brother is equal to 17.

Then a+b= 17.

Get: a= 13 b=4.

So the elder brother's age is 13, and the younger brother's age is 13-4=9.

2. 12 weeks

Suppose each cow eats a dose of grass every day, the grass grows every week, and finally 2 1 cow can eat for c weeks.

Grass in this pastoral area can be fed to 27 cows for 6 weeks or 23 cows for 9 weeks.

Because the number of grass is increasing at a constant rate.

Then 9-week grass is 3 weeks longer than 6-week grass.

Then 23 * a * 9-27 * a * 6 = 3 * B.

B = 15A-( 1)

Compared with the grass in the sixth week, the grass in the third week also grew for (C-6) weeks.

Then 21* a * c-27 * a * 6 = (c-6) * b-(2)

Substitute (1) into (2)

Get 21* a * c-27 * a * 6 = c *15 * a-6 *15 * a.

Divide both sides by a at the same time.

Get 21* c-27 * 6 =15 * c-6 *15.

Get C= 12.

(* is the multiplication symbol)