By what percentage the volume of the cube will decrease if the length of each edge is reduced by 20%.

The length of the edge of the cube was equal to the 1st integral part, which means that its volume was equal to:
1) 1 * 1 * 1 = 1 (part of the volume)
2) 100% – 20% = 80 (%) of the original, the length of the edge of the new cube in percent.
3) 1 * 80% / 100% = 1 * 0.8 = 0.8 (part) of the original edge length of the new cube.
4) 0.8 * 0.8 * 0.8 = 0.512 (part) of the original volume of the new cube.
5) 1 – 0.512 = 0.488 (part) of the original, the volume of the new cube has decreased in comparison with the original.
6) 0.488 * 100% = 48.8 (%) less volume of the new cube.