Adding Stack & Queues exericise question

This question is interesting because of the complexity and the confusion it
causes.
I got the resolution by fluke. I thought Since Double wrapper object is 24N
and since Node object has 2 more Node objects which will also have 24N Double
wrapper items, I thought the answer should be 24N * 3 = 74N.

But the resolution is completely different. Double item is just 24 and not 24 N.
But since we have Node object inside we calculate the total memory cost is
24 + (sum of all the objects & references in the class) which is 24 + 72N.
Very lucky guess.