- The answer

To make a perfect cube, Leena requires another 750 cubes of dimension 10 cm x 10 cm x 10 cm.

- How this was worked out

To begin with, we need to work out the total number of cubes required to make a cube that is 100 cm long, wide and deep.

This can be done by working out how many cubes are needed to make this measurement, which can be done by dividing 100 by 10, giving us an answer that 10 cubes are needed to make each length.

As there are three different measurements needed to make the cube (the length, width and depth), we know that this answer of 10 must be needed for each, and so to work out the quantity of cubes needed to make the entire thing we must work out 10 x 10 x 10, giving us an answer of 1000 cubes.

The next thing that we must find out is the number of cubes that Leena has already used; we can do this by multiplying the 10 from the length with the number of cubes required to make the width 50 cm (which, using the same method of division as above, we can see is 5) and the length 50 cm, giving us 10 x 5 x 5, which equals 250 cubes.

All that is left for us to do is to see how many cubes we need to finish the cube, and this can be done by taking 250 from 100, giving us our final answer of 750 cubes