E. coli is grown for many generations in a medium with ¹⁵NH₄Cl as the only source of nitrogen, so…

Generations. $80\ \text{min}$ at one division every $20\ \text{min}$ is $4$ generations, so the one molecule becomes $2^{4} = 16$ molecules.

(b) Hybrid and light. After the first generation both molecules are hybrid: each has one $^{15}\text{N}$ strand and one new $^{14}\text{N}$ strand. From then on, each heavy strand stays in one hybrid molecule, and all the other molecules have two light strands. So there are always $2$ hybrid molecules, and $16 - 2 = 14$ light ones.

(c) Percentage hybrid. $\dfrac{2}{16} \times 100\% = 12.5\%$, so hybrid : light $= 1 : 7$.

(d) Back to $^{15}\text{N}$. After $40\ \text{min}$ ($2$ generations) there are $2$ hybrid and $2$ light molecules. One more generation in $^{15}\text{N}$ pairs every old strand with a new heavy strand. Each hybrid molecule gives one heavy molecule ($^{15}\text{N}$ with $^{15}\text{N}$) and one hybrid. Each light molecule gives two hybrids. That is $2$ heavy and $6$ hybrid out of $8$: $25\%$ heavy and $75\%$ hybrid, with no light DNA.