If the average score of girls is X and the number of girls is Y, the average score of boys is x( 1-20%)=0.8x, and the number of boys is Y (1+20%) =1.2y.

xy+0.8x * 1.2y = 65 *(y+ 1.2y)

x+0.96x= 143

1.96x= 143

x=72.96

The average score of girls is 72.96.