By the situation of the problem, you can consider that the independent quantity is time (in this case in minutes) and the dependent quantity is the number of envelopes.
In order to complete the table you take into account that Noah stuffs 4 envelopes each minute. This can be written as:
f(x) = 4x where f(x) is the number of envelopes for x minutes
Thus, for the given independent quantities (minutes) you have:
f(5) = 4(5) = 20
f(10) = 4(10) = 40
f(15) = 4(15) = 60
f(20) = 4(20) = 80
f(25) = 4(25) = 100
The previous results are dependent quantities in the table respect to the independent quantities.
For the first three rows you can chose some values for the minutes lower than 5, as for example 1,2,4. Then, you obtain:
f(1) = 4(1) = 4
f(2) = 4(2) = 8
f(3) = 4(3) = 12