Question 1015288
2m×20cm×10cm is cut on small blocks of 10cm×5cm×4cm

2m=200cm, so it's really

200cm×20cm×10cm is cut on small blocks of 10cm×5cm×4cm

The first block has a volume of 40,000 cubic cm.
The second block has a volume of 200 cubic cm.

The first block contains volume equal to 200 of the second.

The second sized blocks have 6 faces 2*((10*5)+10*4)+(4*5)) = 210 sq cm (you see, a rectangular solid has 2 of each of the 3 sized faces, so 3 multiplies, then times 2)

210 sq cm each times 200 blocks is 42,000 sq cm.