Oxygen content of a pond the oxygen content t days after

Question

Question: OXYGEN CONTENT OF A POND The oxygen content t days after organic waste has been dumped into a pond is given by

f(t) = 100(t² + 10t + 100)
             (t² + 20t + 100)

Percent of its normal level.

a. Show that f(0) = 100 and f(10) = 75.

b. Use the intermediate value theorem to conclude that the oxygen content of the pond must have been at a level of 80% at some time.

c. At what time(s) is the oxygen content at the 80% level?