Answer:he started with 35 posters
Explanation:
Let x represent the number of posters that he started with.
Stefan bought 5 posters to add to his collection. This means that the number of posters at this point is
x+ 5
A week later half of all his posters were lost during a move. This means that the amount of posters that were lost is (x+5)/2
The amount of posters left would be x - (x+5)/2
There are now only 15 posters left. This means that
x - (x+5)/2 = 15
Multiplying through by 2
2x - x - 5 = 30
2x - x = 30 + 5
x = 35