450
Camila started with 450 cards. She gave Erin half of them (225) and her brother a sixth of them (75). She was left with 150 cards (450 - 225 - 75 = 150).
If x = number of cards Camila started with, then x/2 = half the cards and x/6 = one sixth of the cards. 150 = number of cards left.
This can be represented as:
x - x/2 - x/6 = 150
Multiply by 6 and solve for x
6x - 3x -x = 900
2x = 900
x = 450 = number of cards Camila started with
