有多少條腿

here is a bus with 7 girls inside,  (車上有 7個女孩 ) Each girl has 7 bags, (每個女孩有 7個包 ) Inside each bag, there are 7 big cats, ( 每個包裡有7 隻大貓) Each big cat has 7 small cats, (每隻大貓還有 7隻小貓) All cats have 4 Legs each! ( 每隻貓都只有4條腿 ) Question: How many Legs are there inside the bus? ( 請問車上有多少條腿)

註：此題不是腦筋急轉彎，所以不要想到奇奇怪怪ex女生有三隻腿還是小貓在肚子裡奇怪的答案....就很單純很單純的數學題目

好厲害的題目，花了我將近２０分鐘咧～

---