Answer:
5 months
Explanation:
Let the number of months be "x"
Lets find equation for both Harry's and Fiona's scheme.
Harry:
80 fixed cost
19 per month
So that would be 80 + 19x
Fiona:
no fixed cost, only 35 per month
So that would be 35x
Now we equate both since we want cost to be same and find x, number of months at which this will occurs. Shown below:

Thus, after 5 months, cost will be same.