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The highest total is 83. The largest numbers 4, 5 and 6 must go on the very top to make the total as large as possible.
The lowest total is 64. Now, the largest numbers must go at the very bottom to have fewer faces visible.
To get 75 the ladder should be made of 4 on the top, 1 underneath, 2 at the bottom. Next column, the 3 at the top, the 5 at the bottom. Finally 6 on its own.
To find this solution, I cut pieces of paper with the number of visible sides on (5, 3, 3, 4, 4, 2), and pieces of paper with the numbers 1, 2, 3, 4, 5, and 6. I created pairs, multiplied them and add the total. I kept changing the pairs around until I found 75.