SOLUTION: a boy is 5 years old, and his sister is twice as old. when the boy is 8 years, what will be the age of his sister?

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the boy is 5 years old.
the sister is twice that old, that is, 10 years old.
the boy will be 8 years in 8-5 years, or in three years.
After three years, the 10 year old sister will become 13 year old.