*5% in decimal form is 0.05.
After one week we have:
N=400*1.05 beetles
After two weeks we have:
N=(400*1.05)*1.05 beetles
After three weeks we have:
N=((400*1.05)*1.05)*1.05 beetles
We can write this as:
Week 1:

Week 2:

Week 3:

From this we can make conclusion that formula for number of beetles after w weeks is given by:
