A fixed unique-read source shows how read length changes coverage. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
The unique-read source stays fixed
The duplicate filter gives 24 unique reads. Only read length changes across the scan.
30−6=24
Shorter reads can miss the coverage gate
With read length 25, coverage is three-halves x, below the minimum 3, so the accepted bit is 0.
24⋅25/400=23<3
The middle read length reaches the boundary
With read length 50, the same unique-read source gives exactly 3x coverage and accepted bit 1.
24⋅50/400=3≥3
Longer reads pass by the same source arithmetic
Read length 75 gives coverage nine-halves x and accepted bit 1.