Make a Venn diagram of 3 groups. Place 8 in the intersection of all 3 group. Your goal is to put numbers into the intersection of two groups as follows:

8% watched all three sports, 10% watched gymnastics and football;

So 2% watched gym and football but not baseball

then figure the numbers in each group without intersections. finally, subtract the sum of each number from 100 to determine the number who watched no sports.