MATH PUZZLE: Can you replace...

Husband and wife are waiting at the bus stop with their nine children. A blind man joins them after a few minutes. When the bus arrives, they find it overloaded and only the wife and the nine kids are able to fit onto the bus.
So the husband and the blind man decide to walk. After a while, the husband gets irritated by the ticking of the stick of the blind man as he taps it on the sidewalk, and says to him, "Why don't you put a piece of rubber at the end of your stick? That ticking sound is driving me crazy."

The blind man replies, "If you would've put a rubber at the end of YOUR stick, we'd be riding the bus ... so shut up."