Question

if if equally was allowed to grow up for 80 minutes then what would be the presence of light and hybrid density is DNA molecule

Answer 1

E coli bacteria divides in every 20 min so in 80 min it will be four generations. 
N15 isotope is present in the environment than shifted in N14 environment, so it will result in the presence of N15 and N14 strands in DNA. 
In the second generation, half will have the previous hybrid bacteria and half will have the light bacteria. 
In the third generation, hybrid generation will again cut half, this time 25% hybrid and 75% light
In fourth generation, 16 copies will be hybrid and 14 will be light