14*
Assuming the girls do not have birth defects, have not lossed any limbs due to lower limb amputations, flesh eating infections, accidents or other freaks of nature. Further assuming that there are not other humans or animals on the bus as the question facts do not indicate that there are. Also there is no indication that the backpacks are in the bus.
However, if the backpacks were in the bus, and assuming the facts in the foregoing paragraph, my answer would be as follows:
(7 girls x 2 legs = 14 legs on all humans on the bus)
=14
(7 big cats x 4 legs = 28 legs in each backpack for the big cats)
(7 girls x 7 backpacks = 49 backpacks)
49 backpacks x 28 legs
= 1372 legs on the big cats
(49 backpacks x 7 little cats = 343 big cats)
(for every big cat there are 7 little cats)
343 x 7 =2401 little cats)
(2401 little cats x 4 legs = 1372 legs on the little cats
= 9604 on the little cats
1372 legs on the big cats
9604 legs on the little cats
14 legs on the little girls
10990 total legs
this took me about 3 hours. thank you very much.
Eddie