96 square units
Assuming the small cubes are 1*1*1.
64 cubes makes a larger cube 4*4*4.
One side has a surface area of 16 (4*4).
Six sides on a cube grants a surface area of 96 (4*4*6).
Each corner cube only exposes three sides, the rest being hidden inside the cube.
The exposed surface area of those 8 corners is 3 each, for a total of 24 (1*1+1*1+1*1)*8 = 24.
Removing those corners, removes 24 units of surface area, but also exposes three new sides, adding 24 units of surface area, for no net change.
So after removing the corners, the surface area remains 96 square units.

## Surface Area of Cube

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