According to the text, E. coli divides every 20 minutes.

• At 0 min: DNA is 100% heavy .
• At 20 min (Generation I): DNA is 100% hybrid because the N strands separate and act as templates for new N strands .
• At 40 min (Generation II): The hybrid DNA replicates in N medium. 50% of the progeny are hybrid and 50% are light .
• At 60 min (Generation III): The cells divide again. The hybrid molecules produce equal numbers of hybrid and light DNA. The light molecules produce only light DNA.
• Starting with 4 molecules from 40 min (2 Hybrid, 2 Light).
• They double to 8 molecules: The 2 Hybrid yield 2 Hybrid + 2 Light. The 2 Light yield 4 Light.
• Total: 2 Hybrid + 6 Light.
• Ratio: 6 Light to 2 Hybrid, or 75% Light and 25% Hybrid.