Coverage is a ratio of checked read bases to 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
Reads become a base ledger
30 reads at 100 bases per read give 3000 read bases.
30×100=3000
Coverage compares read bases to the toy genome
Against a 1000 base toy genome, coverage is 3 times.