Hi,
20,000 bacteria are present in a culture 2 hours after beginning with 15,000 bacteria. that is 5,000 new bacteria in 2 hours
Setting up the proportion, letting x represent the amount of growth in 15hr

cross multiply to solve
2x = 15*5000
x = 37,500 growth in 15hr
Total bacteria: 15,000 + 37,500 = 52,500