Let's see if I calculate this right.
The first bus has 20 boys and 30 girls.
The second bus has a ratio of 5:6, therefore, 24 boys.
Since it's 50-50, there are 24 girls, too.
The third bus has a ratio of 7:8, so 21 girls. 60% of the 3rd bus are girls, so
.60x=21, x=35 altogether. 21 girls and 14 boys on the 3rd bus.
So, total there are 20+30+24+24+14+21=133