A bacteria culture starts with 900 bacteria and doubles in size every half hour.
(a) How many bacteria are there after 4 hours?
(b) How many bacteria are there after t hours?
(c) How many bacteria are there after 40 minutes? (Round your answer to the nearest whole number.)
(d) Graph the population function and estimate the time for the population to reach 40,000. (Give your answer to the nearest tenth.)