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A math problem about age
Solution: Let the age difference between elder brother and younger brother be X.

My brother said, "When I was your age this year, you were only 5 years old." You can know that my brother is my brother's age this year. My brother has lost for X years, so my brother is (5+X years old) this year. My brother said, "When I grow to your age this year, you will be 17 years old." We can know that my brother will be X years old when he is a brother this year, so this year my brother is (17-X years old).

Solution: Let the age difference between elder brother and younger brother be X.

17-X-X=5+X (to illustrate, you can't use x alone at the beginning, so we use elder brother's age-age gap = younger brother's age or younger brother's age+age gap = elder brother's age, and I use elder brother's age-age gap = younger brother's age).

3X= 17-5

3X= 12

X= 12÷3

X=4

Elder brother's age: 17-4= 13 (years old)

Younger brother's age: 5+4=9 (years)

A: My brother 13 years old, and my brother is 9 years old.