Greetings from Brasil...
Let Ca be the function for Plan A and Cb the function for Plan B.
According to the problem statement, we can conclude that:
Ca(n) = 3n + 8
Cb(n) = 5n
where C is the cost and n is the number of videos
Cost that the user will pay for having watched n videos
Ca = cost of Plan A
Cb = cost of Plan B
Is there any point (n) that Ca will be equal to Cb??? Let's equate the functions to check if there is a common point and if there is, what would it be.
Ca = Cb
3n + 8 = 5n
2n = 8
n = 4
In n = 4 we have a condition for the cost that does not matter Ca or Cb.
I will present 2 ways to solve:
1 - building the graph of the functions and observing the points where the Y axis (cost axis) its smaller
2 - assigning 2 points - below n = 4 and above n = 4 - to observe the cost behavior
1 - see attachment
2 - let's choose the points n = 1 and n = 10 (as already said, n=4 does not matter Ca or Cb - meeting point / intersection point)
n = 1
Ca(1) = 3.1 + 8 = 11
Cb(1) = 5.1 = 5
Ca > Cb, then Cb better to the user
n = 10
Ca(10) = 3.10 + 8 = 38
Cb(10) = 5.10 = 50
Ca < Cb, then Ca better to the user
We can come to the following conclusion:
0 < n < 4 → better use Plan B (in this interval the cost to the user will be lower with Plan B)
n = 4 → It doesn't matter to use Plan A or Plan B.
n > 4 → better use Plan A (for n above 4 the cost to the user will be lower with Plan A)