DAY 26

Comment

Imagine starting to fill the box with 5 mm marbles.
How many marbles would you need to 'cover' the base of the box?

Answer

27.

Readers' methods

Magnus W, Gothenburg, Sweden:
A marble of width 5 mm fits in a cube with the side 5mm.
To cover the bottom of the 15 mm cube, you need 3x3=9 of these small cubes, and once the bottom is covered, you have just used one third of the height in the big box. Thus you can continue with two more layers, each containing 9 cubes (=marbles).
So the big box will contain 3x9=27 small marbles.