(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.