I'm sorry for the late response, but maybe it will still be useful to you.

In this case the lower bound is 550 g, and the upper bound is 600 g, because you know that the mass of the car lies somewhere between 550 and 600. We know it can't be 600, because the scale falls at that point instead of balancing. (The question is a little unclear on whether the mass can be 550, but I will assume that it is not.) Because he doesn't have any smaller weights, he can't determine the weight of the car to any greater degree of accuracy than that.

The way you graph this on a number line is you draw the line segment from 550 to 600, but instead of closed circles on the endpoints, you draw open circles. Closed circles would mean that the mass could be 550 or 600 in addition to all the points between them. But we already decided that the mass can't have those values, so we draw open circles to indicate that it can be anything between them, but not the endpoints themselves.