Coverage is a ratio of counted read bases to genome bases, not a standalone label. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Ten reads make one-times toy coverage
10 reads at 100 bases each make 1000 read bases against a 1000 base toy genome.
10⋅100=1000,1000/1000=1
Twenty reads make two-times toy coverage
20 reads keep the read length at 100 bases, so read bases become 2000 and coverage becomes 2 times.
20⋅100=2000,2000/1000=2
Thirty reads make three-times toy coverage
30 reads make 3000 read bases. Dividing by the same toy genome gives coverage 3 times.