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I counted manually.

Boys : Girls
3 : 2
0 : 25 (bus 1)
45 : 30
15 : 0 (bus 2)
So,
30 : 30 (after bus 2)

Boys: 30+15 = 45
Girls: 30+25 = 55
Total: 100

No formula I used. Just manually. Great enigma!