Right now Seth's age is the age of his brother Eric. Twenty-one years ago, Eric was twice as old as Seth. What are their ages now?
Let's assume Seth's current age is x and Eric's current age is y.
According to the given information, Seth's age right now is the age of his brother Eric. Therefore, x = y.
Twenty-one years ago, Seth's age would have been x - 21 and Eric's age would have been y - 21.
According to the second piece of information given, Eric was twice as old as Seth twenty-one years ago. Therefore, we can write the equation: y - 21 = 2(x - 21).
Since x = y, we can substitute x for y in the equation: x - 21 = 2(x - 21).
Expanding the equation, we get x - 21 = 2x - 42.
Rearranging the equation, we have 2x - x = 42 - 21.
Simplifying, we get x = 21.
Therefore, Seth's current age (x) is 21 years old.
Since x = y, Eric's current age is also 21 years old.
So, Seth and Eric are both 21 years old now.