E. coli typically divides every 20 minutes, so in 60 minutes there are three rounds of replication. Starting from 10 cells, the population grows: $10\to 20\to 40\to 80$ cells.

After the first division (20 min), each daughter cell contains one heavy (${}^{15}$N) strand and one light (${}^{14}$N) strand. All 20 cells are hybrid.

At each subsequent division, only the cells containing a heavy strand can pass it on; the cells with two light strands remain heavy-free. The number of cells that contain at least one heavy strand stays at 20 throughout subsequent divisions.

So after 60 minutes (80 cells total), $80-20=60$ cells have DNA totally free from ${}^{15}$N.