Answer:
a) The growth of bacteria at nth hour is : 
b) At the end of the fourth hour, there are 81 stands of the bacteria
present.
Step-by-step explanation:
The number of initial bacteria in the dish = 3 strands
Rate at which bacteria multiply = 3 times every hour
So, the general form which represents the growth
As it multiplies three times every hour, so at nth hour:
The growth of bacteria at nth hour is : 
Now, at n = 4, the growth of the bacteria is 
= 3 x 3 x 3 x 3
⇒
= 81
So, at the end of the fourth hour, there are 81 stands of the bacteria present.