Let the number of boys be x.
Then the number of girls is 2x.
The total number of fourth graders is x + 2x = 81.
Combining like terms, we get 3x = 81.
Dividing both sides by 3, we get x = 27.
Therefore, there are 27 boys and 2 * 27 = <<27*2=54>>54 girls in the fourth grade. Answer: \boxed{27,54}.
There are twice as many girls as boys in the fourth grade classes at Joe's school. There are a total of 81 fourth graders. How many boys and girls are there in the fourth grades
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