A7: 0
B7: =B1
C7 (to be copied down): =ROUND(B7*$B$3)
(really it's ROUND(B7*$B$3,0), but that wasn't the point here)
D7 (to be copied down): =B7+C7-E7
E7 (to be copied down):
=if(B7+C7-B7*$B$4>$B$2,$B$2,B7+C7-B7*$B$4)
(ROUND should really be used there, but this wasn't required by
the question)
A8 (to be copied down): =A7+1
B8 (to be copied down): =E7
There are different places at which you could implement the "crowd limit" part, where there can't be more than 5000 (or whatever's in B2) bacteria; but implementing it in the "end" column means that the death number will reflect the crowd limit, and you had to achieve this for full marks.
Distinguishing between relative and absolute references is crucial if the "copied down" formulas are going to be right.