Yeah when I went through it after it was very confusing.
The difference I found with you Family A answer was finding out how old the GP was. Which comes from the statement ' The youngest grandparent is half the age of the oldest grandchild older than the oldest parent' Half the age of the oldest grandchild is 17.5 add that to oldest parent (45) makes it 62.5. That lead to you being out. As for B I need to look through more but one third of 87 is exactly 29 though so maybe some rounding errors for you made it more confusing.
Here are my A solutions:
Sorry for my messy work I was going to type it up in a few days.