Yuan is 7 years older than his brother David and twice his age. How old is David?
Let x be David's age. Yuan is seven years older than David, 7 + x = Y years.
Yuan is also twice David's age, 2 * x = Y years.
Since these two equations equal the same thing, we can write: 7 + x = 2 * x.
Subtracting x from both sides we get 7 = x, so David is 7 years old.
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