I think you didn't quite copy the question correctly...it's hard to understand what the problem is.
An upper bound is the "tipping point", so to speak: it's a value which is greater than or equal to everything else in the set. For this problem it sounds like the upper bound is 600 g. Since you can put anything up to (but not including) 600 g on the scale without it tipping, we call 600 g the upper bound (actually it is the least upper bound).
A lower bound is the opposite. It sounds like the lower bound is 50 g, but it's hard to tell from the problem. If Daniel wants to use at least 50 g, then anything above or equal to 50 g is allowable, but anything lower is not. We call 50 g the greatest lower bound.