28 players.
We know that there are seven teams with seven players each. This is 49 players total. We know that the fraction 3/7 must be equal to x/49. Since we multiples the denominator by 7, we have to multiply the numerator by 7 too. This is 3 x 7 =21. Therefore, 21 out of the 49 players were disqualified. However, the question wants to know how many qualified, so we must subtract. 49-21=28 players. Hope this helped!