I really need help understanding optimization, so if someone could explain it to me, i'd be grateful.

The number of bacteria in a culture at time t is given by

N= 1000(25+te to the -t/20)

for 0 is less than or equal to t, which is less than or equal to 100.

a) Find the largest and the smallest number of bacteria in the culture during the interval.

b) At what time during the interval is the rate of change in the number of bacteria a minimum?

Thanks.