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.
We can figure this out using the explicit formula.

n represents the term we are looking for.f(1) represents the first term in the sequence, which in this case, is -242.d represents the common difference, which in this case, is -9.
f(n) = -242 + -9(n - 1)
f(n) = -242 - 9n + 9
f(n) = -233 - 9n
Now, we can input 28 for n and solve.
f(28) = -233 - 9(28)
f(28) = -233 - 252
f(28) = -485
The 28th term of the sequence is -485.
Serach it, Thats what I did!