CHALLENGE 1
A\ The highest number of cubes you can calculate is 83 in total, looking like this:
6 6 665544 6
25 52 2
314 413 3
B\ The lowest number of cubes you can calculate is 64 in total, looking like this:
1 1 113322 1
53 35 5
462 264 4
C\ We calculated a&b by making the cubes for each possible solution. We did this because it will be easier to get the solution if you can visualise the answer. Also we made sure that the higher number had more surface area showing to make a higher total. For the lowest total we made sure that the higher number didn't have much surface area showing and the lower number had more surface area showing making a lower total.
D\ This is how to make the total of 75:
5 5 553311 5
43 34 4
621 126 6