Coverage is a physical readout count divided by a toy genome length. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Read bases add before dividing
The toy run has 20 reads of length 100 bases, so total read bases are 2000.
Nbases=20⋅100=2000
Coverage is read bases over genome bases
Dividing by a genome length of 1000 bases gives coverage 2 times.