According to the text, E. coli divides every 20 minutes. In 60 minutes, the cells undergo 3 generations (60/20 = 3).

1. Total Cells: Starting with 10 cells, the number increases to cells.
2. DNA Composition: The initial DNA is fully heavy , providing 20 heavy strands (10 cells 2 strands). In semi-conservative replication, these original heavy strands are conserved and serve as templates.
3. Distribution: In the presence of N, newly synthesized strands are light. The 20 original N strands will be part of 20 hybrid DNA molecules, found in 20 cells.
4. Free from N: The remaining cells will contain only light DNA . Calculation: Total cells (80) - Cells with N (20) = 60 cells totally free from N.