90 / 10 = 9
1. 9, Lorena should buy 9 packs of biscuits to get 90 pieces of it.
90 / 15 = 6
2. 6, Lorena should buy 6 packs of biscuits to get 90 pieces of it.
90 / 20 = 4 and 1/2
3. 4.5 or 4 and 1/2, Lorena should but 4 and half of a pack biscuits to get 90 pieces of it.