Answer:
Ok, suppose that there are M Petri dishes, where M is an integer number.
If there are N students in the class, then the number of Petri dishes that each student gets is:
# = M/N.
(always that N divides M)
If not, we will have a remainder, R, of Petri dishes such that:
M ≥ N
R < N
This means that:
The total number of Petri dishes must be equal or larger than the number of students.
If M is larger than N, and N does not divide M, then the remainder of the quotient will be smaller than N.
Now with this, you can replace the numbers and do the math.
"The more students that decide to do the bacteria experiment, the fewer the number of Petri dishes each student gets."
You can easily see this because the equation for the number of Petri dishes that each student gets is:
# = M/N.
Here, N is in the denominator, then if the number of students in the class increases, the denominator in the operation increases, which means that the quotient will be smaller.