The width of the cupboard fits the width of two boxes exactly.

0.9m = 90cm

So 2 widths of a box = 90cm

There are 4 boxes horizontally stacked on top in Figure B.

Since 4 ÷ 2 = 2,
4 box widths = 2 x 90cm = 180cm

Notice that in both Figure A and Figure B there are 2 boxes stacked vertically at the bottom.

So the difference between the arrangements is that the 4 boxes on top are stacked vertically in A, and horizontally in B.

In A the gap is 42cm while in B, the gap is 10cm.

42cm - 10cm = 32cm. So we can see that 4 box widths is 32cm longer than 2 box lengths.

2 box lengths = 180cm - 32cm = 148cm
Then 1 box length = 148cm ÷ 2 = 74cm

Gap in Figure B = cupboard width - 1 box length

= 90cm - 74cm
= 16cm


b)

height of cupboard (based on Figure B)
= 4 box widths + 1 box length + 10cm gap
= 180cm + 74cm + 10cm
= 264cm
= 2.64m


Or

height of cupboard (based on Figure A)
= 3 box lengths + 42cm gap
= 3 x 74cm + 42cm
= 222 + 42 cm
= 264cm
= 2.64m