I hope you don't find the solution too messy. If there is anything you don't get. Let me know and I will explain it:
I think its the second one... (hope its right)
The answer is kinda hard and easy so I will explain how to do it but not answer. So first you divide 105 by 7 to get the number of pounds of food per snake. Then you times it by 3 for 3 snakes, by 5 for 5 snakes, and by 10 for ten snakes. At least that is what I think. Hope this helped!!