(38 - 15) * 2 = c
c = original cards.
38 - 15 = 23
23 * 2 = 46
You started with 46 cards.
38 - 15 represents subtracting the cards you bought from the half of your original cards.
The * 2 represents multiplying the number of cards you had (without the new ones you bought) being multiplied by 2 (because you lost half of them).
