Explanation:
the volume of a box is always
length × width × height
in our first case here :
11 × 6.4 × 3 = 211.2 cm³
the volume of the small boxes is
3 × 2 × 1 = 6 cm³
the volume of the large box is
15 × 12 × 10 = 1,800 cm³
so,
1800/6 = 300 small boxes will fit into the large box.
because the dimensions of the small boxes are factors of the dimensions of the large box (3 is a factor of 15, 2 is a factor of 12, 1 is a factor of 10), it will be a perfect fill without any unused leftover pockets of space.
so, truly 300 small boxes will fit into the large box.