I've already been complemented twice on my previous answer, but then I discovered that I mis-read the question. My entire original answer was wrong, and I have to delete it.
I don't believe that any number can satisfy both of those conditions.
I'll say the question has no answer.
Using the proportion given, there will be 57 catfish in the pond.
The proportion given to us is
5.7/20 = x/200
Cross multiply:
5.7*200 = 20*x
1140 = 20x
Divide both sides by 20:
1140/20 = 20x/20
57 = x
99
/ \
11 9
/ \
3 3
165
/ \
33 5
/ \
3 11
GCF=3
For this case we have to:
There are 6 plants per tray.
There are 6 trays for each flat, so for each flat there are 
plants.
Saturday day:
Thus, on Saturday they sold 648 plants.
Sunday: 
Thus, on Saturday they sold 756 plants.
Thus, in total 
plants were sold.
Answer:
plants were sold.
There are an infinite number of possibilities, and not enough information
to decide which possibility is really the one inside the function machine.
Here are a few. Each of these gives the result that you described,
and there are an infinite number of others:
f(x) = x
f(x) = 2x + 1
f(x) = 10x + 9
f(x) = x² - 2
f(x) = 7x² - 8
f(x) = 31x³ + 30
f(x) = log( |x| ) - 1
f(x) = ln( |x| ) - 1
f(x) = x tan(45°)
.
.
etc.
