I don’t understand the solution to number 17 from the section counting and listing on pg 84 of the math guide. Can you please explain it to me?
First take care of the restrictions. If Alicia’s mother and father need to be on opposite ends, then she has 2 choices for the first slot. Once she makes a choice for that slot, though, there’s only one choice for the last slot.
Once mom and dad are placed, the other three family members can go in any order in the middle three slots. So there are 3 choices for the second slot, then 2 choices for the third slot because one picture was placed in the second slot, and the only 1 choice for the fourth slot because there’s only one picture left to place.