Following the standard equation for CSMA/CD :
T.T. = 2 * P.T.
==> (5000 * 8 * 210) / (20 * 106) = (2 * 20 * 103) / x
==> x = (20 * 2 * 20 * 109) / (5000 * 8 * 210) m/s
==> x = (8 * 1011) / (4 * 104 * 210)
==> x = 1011 / (104 * 29)
==> x = 10000 / 512
==> x = 19.53 km /s
Hence the required speed is 19.53 km/s..The main issue is whenever we consider something related to system like packet size etc. so we take powers of 2 in such cases..And for something related to channel and not system like bandwidth etc are taken in powers of 10..
Reference of Unit System Consideration : Petersen Davie on CN