There  are  23 phage plaques derived from a 0.1 ml aliquot (sample) of
   the 10^5 dilution.
<LI> To calculate the initial undiluted viral density (particles / ml):+ (23 * 10^5 ) / 0.1 ml = 23 * 10^6 / ml, or+ 2.3 * 10^7 / ml