In the air over a particular region at an altitude of 500 m above the ground, the electric field is 150 N/C directed downward. At 600 m above the ground, the electric field is 80 N/C downward. What is the average volume charge density in the layer of air between these two elevations? the answer is supposed to be in C/m^3. i have looked at this question for a long time now and can not find a way to solve it. i think this problem would be easier if the volume of the slab of air of 100m height can be calculated and then use Gauss' law. but other then that i can not think of any other way to solve it. can anyone help me?