The ralation between propagation speed and bit rate

Would you mind intepreting the ralation between propagation speed and bit
rate for me?
In fact, these days I am always pondering this question, however it puzzles
me deeply.

The following is a example:
A 1-km-long, 10-Mbps CSMA/CD LAN(not 802.3)has a propagation speed of
200m/um. Repeaters are not allowed in this system. Data frames are 256 bits
long, including 32 bits of header, checksum, and other overhead. The first
bit slot after a successful transmission is reserved for the receiver to
capture the channel in order to send a 32-bit acknowledgement frame. What is
the effectivedata rate ,excluding over head, assuming that there are no
collisons?