A culture of E. coli bacteria, with its DNA completely labeled with...

The generation time of E. coli is 20 minutes.

Number of generations in 80 minutes = $\dfrac{80}{20} = 4$.

Total number of DNA molecules after 4 generations = $2^4 = 16$.

Initially, the DNA is completely heavy ($^{15}\text{N}-^{15}\text{N}$), containing 2 heavy strands.

During replication in a $^{14}\text{N}$ medium, these 2 heavy strands separate and act as templates, forming exactly 2 hybrid DNA molecules ($^{15}\text{N}-^{14}\text{N}$) in all subsequent generations.

The remaining DNA molecules will be completely light ($^{14}\text{N}-^{14}\text{N}$).

Number of light DNA molecules = $16 - 2 = 14$.

Ratio of light DNA to hybrid DNA = $14 : 2 = 7 : 1$.

Answer: $7 : 1$