Bounces among two opposing ant colonies

Alice releases her $40$ ants at one end of a thin string and Bob releases his $80$ ants at the other end, all walking toward the opposite end. When two ants meet they both reverse direction. How many collisions take place in total?

3200. Treat a collision as two ants passing through each other and swapping identities; the set of positions over time is the same. Then every Alice ant meets every Bob ant exactly once and no other meetings occur, giving $40\times80=3200$.