\(R =3 r\)
Potential energy of smaller drop :
\(U _{1}=\frac{3}{5} \frac{ kq ^{2}}{ r }\)
Potential energy of bigger drop :
\(U =\frac{3}{5} \frac{ kQ ^{2}}{ R }\)
\(U =\frac{3}{5} \frac{ k (27 q )^{2}}{ R }\)
\(U =\frac{3}{5} k \frac{(27)(27) q ^{2}}{3 r }\)
\(U =\frac{(27)(27)}{3}\left(\frac{3}{5} \frac{ kq ^{2}}{ r }\right)\)
\(U =243 U _{1}\)