Answer:
Let's suppose that one cubic foot of marble is x.
20x = 3400
But because the weight has a range of +-100, then 20x is between 3300 and 3500. Write it like this:
3300<20x<3500
Divide by 20:
165<x<175
The meaning of this is that one cubic foot of marble weight between 165 and 175 pounds