Water droplet problem/electric fields

[jh12]

We just went over electric fields in class and I need help starting a problem:

A water droplet is stationary and has radius 0.0200mm. The Electric field where the droplet is located in is 150 N/C downward. How many excess electron charges must the water droplet have?

 

[Andy Resnick]

One clue is simply look at the units for electric field. The upward force from the field on the charges must balance the downward force from gravity.