Ten \( E. coli \) cells with \( ^{15}\text{N-dsDNA} \) are incubated in medium containing \( ^{14}\text{N} \) nucleotide. After 60 minutes how many \( E. coli \) cells with have DNA totally free from \( ^{15}\text{N} \)?

E. coli divides every 20 minutes. In 60 minutes, 3 generations occur. Total cells = \( 10 \times 2^3 = 80 \). The initial 10 cells retain 1 heavy strand each (hybrid DNA). Thus, cells with totally free DNA = \( 80 - 10 = 70 \). Wait, looking at options, let's re-evaluate standard formula: total hybrid strands = 20, total DNA molecules = 160, hybrid molecules = 20, free molecules = 140... wait, total cells = 80, out of which 10 have hybrid DNA, so 70? Let's check options: 60, 80, 20, 40. Actually, if 1 cell starts, after 3 generations it forms 8 cells, out of which 6 have free DNA. For 10 cells, \( 10 \times 6 = 60 \) cells.