Every earlier coverage lesson fixed genome length at a thousand bases and scanned read count or read length; this scan holds those fixed and scans genome length itself. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Every earlier coverage row held genome length fixed at a thousand
Every earlier coverage lesson in this book kept genome length at 1000 bases and only ever varied read count or read length. Here read count stays 20 and read length stays 100, but genome length itself drops to 500 bases, giving coverage 4 x.
50020⋅100=4
Genome length itself is a source factor, not just the reads
Read count and read length never change across these rows. Genome length alone rescales the checked coverage ratio, the same read-base budget divided by a different denominator.
100020⋅100=2
A larger genome lowers coverage for the same read budget
At genome length 2000 bases, the same read-base budget gives coverage 1 x. Three rows separate genome length from the read-count and read-length factors already scanned elsewhere in this book.