Answer:
If a cell at metaphase of mitosis contains 20 sister chromatids, it means to say that the cell has 10 chromosomes.
During the metaphase of mitosis, the chromosomes are aligned. The set of two sister chromatids generates a mitotic chromosome.
Chromatide is a longitudinal unit of the already duplicated chromosome, it joins its sister chromatide by the centromere, this is observed in the phases of prophase and metophase