In January, the ratio of the number of students in 6Gr to the number of students in 6Rp was 3:4. In March, 6 students from 6Gr joined 6Rp, the ratio of the number of students in 6Gr to the number of students in 6Rp became 5:9. Find the number of students in 6Gr at first.
Let the number of students in 6Gr in January be 3x.
The number of students in 6Rp in January would be 4x.
In March, after 6 students from 6Gr joined 6Rp, the number of students in 6Gr became 3x - 6, and in 6Rp became 4x + 6.
Accordingly, the ratio of students in 6Gr to the number of students in 6Rp became (3x - 6)/(4x + 6) = 5/9.
Cross-multiplying, we get (3x - 6) * 9 = 5(4x + 6).
Simplifying, we get 27x - 54 = 20x + 30.
Combining like terms, we get 7x = 84.
Dividing both sides by 7, we get x = 12.
Substituting this value back into the expression for the number of students in 6Gr, we get 3x = 3 * 12 = <<3*12=36>>36.
Therefore, the number of students in 6Gr at first was 36. Answer: \boxed{36}.