Answer:
The correct answer is n = 20.
The term diploid refers to the presence of two sets of chromosomes in the cell, usually one inherited from the mother and one from the father.
The number of chromosomes in a diploid cell (or somatic cell) of any organism is exactly equal to the twice of number of chromosomes found in its gamete or haploid cell.
This is why the diploid number of chromosomes is expressed as 2n and haploid number of chromosomes is expressed as n.
For example, 2n or diploid number of chromosome of human cell = 46 while haploid number(n) of chromosome found in human gamete = 23.
Similarly, if 2n in standard lab mice = 40 then its "n" would be = 20