bit rate is the count of bits per second.
$80$ $characters$ =====>$100µsec$
$80\times 8$ $bits$======>$100µsec$
$640$ $bits$=======>$100µsec$
$1µsec$=======>$6.40$ $bits$
$1\times10^{-6} sec$=====>$6.40$ $bits$
$1 sec$====>$6.40\times 10^{6}$ $bits$
$bit$ $rate=6.40Mbps$