In example 20.3 in the text, the net force on a 1.0 nC charge located between two 10 nC charges is calculated. How would the answer change if the two 10 nC charges were replaced by two - 30 nC charges, leaving the 1.0 nC charge the same

The forces experienced by the middle particle are attractive, and the net force will remain the same (0) if and only if the distances of the sides particles to the middle particle are the same.

Explanation:

In example 20.3 the forces experienced by the middle particle are repulsive because all the particles are positive, for the case in which the particles on the sides are replaced for negative charge particles the forces experienced by the middle particle are attractive. Regarding the net force, because we don't know the distances we can not give a definitive answer, what we can say is that if the distances from the middle particle to the sides particles are the same the net force is zero for both cases (remain unchanged).