Explanation:
x = number of stamps she had at the beginning
x/2 + 18 = 47
you see ? she sold half of her original stamps, which left her with the other half.
and then she bought additional 18.
so, in total she now has half of her original stamps + the 18 new ones, and that total is 47.
x/2 + 18 = 47
x/2 = 29
x = 58
so, she had originally 58 stamps.