physics
posted by ashley  HELP PLEASEE!!! on .
Two point charges +4q and +q are placed 30 cm apart. At what point on the line joining them is the electric field zero?
(a) 15 cm from charge 4q
(b) 20 cm from charge 4q
(c) 7.5 cm from charge q
(d) 5 cm from charge q

The distance to the +q charge must be half the distance to the +4q charge, so that the electric fields cancel. Remember the inverse square dependence upon distance!
That puts you 10 cm from q and 20 cm from 4q.
That happens to be one of your choices. 
the two charges of magnitude 1rc and 2rc are separated by 10cm from each other at what point on the line joining the two charges is the electric field strength zero?