Two charged balloons 2.1 × 10-1 meters apart have a repulsive force of
1.3 × 10-1 newtons. The charge on one balloon is 8.2 × 10-7 coulombs. What is the charge on the other balloon?
(k = 9.0 × 109 newton·meters2/coulomb2)

Question

Two charged balloons 2.1 × 10-1 meters apart have a repulsive force of
1.3 × 10-1 newtons. The charge on one balloon is 8.2 × 10-7 coulombs. What is the charge on the other balloon?
(k = 9.0 × 109 newton·meters2/coulomb2)

anonymous · Answer

Remember Coulomb's Law:$$F=k \frac{ q _{1} q _{2}}{ r ^{2} }$$where F is the force due to charges q1 and q2; k is a constant; and r is the distance between the two charges.

Now look at what the problem gives you.  It gives the distance between the two charges (the two charged balloons); it gives you the force due to those charges; and it gives you the constant; and it gives you one of the charges.  Do you see, now, how to solve the problem?