During the lag time, by definition, bacteria newly introduced into a culture will undergo metabolic changes necessary for use of the medium, but will NOT increase in number. Therefore, at the end of the first 30 minutes elapsed time, the number of bacteria in the culture will be 2 x 10^2. Thereafter, the number of bacteria will double every 20 minutes (the definition of generation time). This means that after 2 1/2 hours of total elapsed time, 6 generations will have occurred, so the original inoculum number is multiplied by 2, 6 times. 6 x 10^2 (1st Choice) is not the correct answer. If this was your answer, you forgot that lag periods occur only once per culture, and also incorrectly multiplied the staing inoculum number by an incorrect number of generations (three). 1.2 x 10^3 (2nd Choice) is not the correct answer, and reflects the common student error of counting the number of possible generations and multiplying by that number. Remember that every generation time means that every bacterium in that culture has divided into two, so you must multiply by two, 6 times or 26. 1.6 x 10^3 (3rd Choice) is not the correct answer, and reflects the common error of forgetting that the lag time occurs only one time per culture. If you picked this answer, you divided the 2.5 hours of culture time between three lag periods and three correctly calculated generations (23).