From 10 parent $E.coli$ cells: 1st generation (20 mins): 10 cells $\rightarrow$ 20 cells (all hybrid $^{15}N - ^{14}N$). 2nd generation (40 mins): 20 cells $\rightarrow$ 40 cells (20 hybrid $^{15}N - ^{14}N$ + 20 cells with $^{14}N$). 3rd generation (60 mins): 40 cells $\rightarrow$ 80 cells (20 hybrid $^{15}N - ^{14}N$ + 60 cells with $^{14}N$). Therefore, after 60 minutes, 60 $E.coli$ cells will have DNA totally free from $^{15}N$.