On the final day I cycled the remaining 5 miles
On Day 5 I started with 15 miles (as I cycled two thirds, leaving 5 miles).
On Day 4 I started with 25 miles (as I cycled 10 miles).
On Day 3 I started with 100 miles (as I cycled three quarters, leaving 25 miles).
On Day 2 I started with 200 miles (as I cycled half, leaving 100 miles).
On Day 1 I started with 400 miles (as I cycled half, leaving 200 miles).
How many squares, of any size, can you find on this chess board which do not contain a Rook?
The original large cube with 16 small cubes along each edge would required 16 x 16 x 16 = 4,096 small cubes.
Removing the outside layer would leave a large cube now with 14 small cubes along each edge (which requires 14 x 14 x 14 = 2,744 small cubes).
It isn't 15 small cubes because you're removing both ends of an edge.
Therefore you have removed 4,096 - 2,744 = 1,352 small cubes.