(5*20+3*60)/(5+3)=35 kg
2. There are 75 girls.
Suppose there are x people in the original * * *,
Then it is found that boys account for 6/ 1 1 and girls account for 5/1;
After adding 5 people, the total number is (x+5), 9/ 17 for boys and 8/ 17 for girls.
So: (x+5) * (8/17)-x * (5/11) = 5.
Solve it, x= 165.
Therefore, the original number of female students is165 * (5/11) = 75.