In a laboratory in Seoul, a stem cell divides every 20 h, and every new cell does the same…

Count the centromeres for the chromosomes and the DNA molecules for the chromatids. The mitotic index is the cells in mitosis divided by all the cells, times $100$. The time of a phase is the share of the cells in it times the length of the cycle. One cell gives $2^n$ cells in $n$ cycles, and $N - 1$ divisions take place in going from $1$ to $N$ cells.