In the given question it is mentioned that Lea bought 7 posters . Half of the posters she collected got burnt by fire a week later. Lea is left with only 18 posters.
let us assume that the number of posters Lea had in the beginning = x
The number of posters she had after fire burn = x/2.
Now we can come to the required equation
x/2 = 18
x = 18 * 2
x = 36
x = 36
So it can be stated that Lea started with 36 posters. the severn posters stated here in the question has no importance at all. It is just there to puzzle.