Question

A cube has side length a. The side lengths are decreased to 20% of their original size. Write an expression in simplest form for the volume of the new cube in terms of a.

Answer 1

Volume of the original cube is a³ .
That's (a · a · a) .

If you decrease the sides to 20% of their original size,
then the new volume is  (0.2a · 0.2a × 0.2a).

That's  0.008a³  or  (0.2a)³ . 

Take your choice.  I can't decide which one is simpler.

Answer 2

20% is mean you have a cube of 80%
So 80%=0.8
That means the volume of the new cube is (0.8)3 I mean multiply 0,8x0.8x0.8 = 0.512a cubic unit