Answer:
If all the dimensions of the rectangular pyramid are multiplied by 1/5, the volume will be 125 times smaller.
Explanation:
A rectangular pyramid has the following dimensions:
length(l), height(h) and width(w)
The volume is:

If all the dimensions of the rectangular pyramid are multiplied by 1/5
Then we have that:

The modified volume will be:

So
If all the dimensions of the rectangular pyramid are multiplied by 1/5, the volume will be 1/125 of the original, that is, 125 times smaller.