Model Approach for this question. With regards to your question on whether to put all the units to 24, you actually don’t know that 1 unit equals to 24 cards, so you cannot do that. At this point, 1 unit is still an unknown so you cannot say 1 unit equals to 24 cards or how many units equals to 24 cards.

Instead what you want to do is that, separate the units from the numbers and handle them independently. In the 1st transfer, referring to my model above, you can say that Perry gave away 1 box (or 1 unit) + 24 cards to Dave. But you cannot say Perry gave away 24 + 24 cards to Dave because at this point, you don’t know how many cards 1 box or 1 unit represents.

In the second transfer, each of this boxes will be split further into half to visualize Dave giving away 3/8 of his boxes + 9 cards to Perry.

Overall, using model approach for this question will be time consuming and troublesome, I strongly recommend using unitary approach to solve this question. And it’s rather difficult for me to explain the model and solution through text since it’s quite complicated :( but do leave a comment if you still need help / clarifications, I’ll try to explain to you again.