2100 bacteria will become 4200 in 10 days, 4200 will become 8400 after 20 days, 8400 will become 16800 after 30 days, 16800 will become 33600. for the final 5 days you will add half of the double period. so half of 33600 is 16800. Now add those together. The answer is 50,400 bacteria after 45 days.
