This is what I found online for you!! Not my words. :))
5% in decimal form is 0.05.
After one week we have: N=4001.05 beetles
After two weeks we have: N=(4001.05)*1.05 beetles
After three weeks we have: N=((400*1.05)*1.05)*1.05 beetles
We can write this as: Week 1: N = 400* 1.051
Week 2: N = 400* 1.052
Week 3: N = 400*1.053
From this we can make conclusion that formula for number of beetles after w weeks is given by:
N = 400 *1.05w
