Steps:
1)Data:
q1=charge of first balloon=1.2 × 10^-6
q2=charge of second balloon=1.2 × 10^-6
Condition:q1=q2
then for convenience,we say q instead of q1 and q2.
Radius=r=5*10^-1
Radius is the distance between balloons!
Magnitude of repulsive force=F=?
2)Solution:
F=kq1q2/r^2
As q1=q2=q
So
F=kqq/r^2
=kq^2/r^2
Putting values:
F=((9*10^9)(1.2 × 10^-6)^2)/(5*10^-1)^2
Using calculator:
F=0.05184 N