You are given two unknown point charges, Q1 and Q2. At a point on the line joining them, one-third of the way from Q1 to Q2 the electric field is zero . What is the ratio Q1/Q2?

Answer 1

E is directly proportional to q
Break the distance into three parts, one between q1,Q2, and the other two parts on the Q2 side.

q1/1^2=q2/2^2
q1/q2=1/4