Coverage arithmetic starts from duplicate-filtered unique reads. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Unique reads create read bases
The duplicate-filter source gives 24 unique reads. Each read contributes 50 bases.
24⋅50=1200
The toy run reaches three-x coverage
Read bases 1200 over genome size 400 gives coverage 3x.