Answer
$8.94\times 10^{-19}$
Work Step by Step
One sphere will have a negative charge while the second sphere will have a positive charge.
According to the given condition:
$F_e=F_g$ (where $F_g$ and $F_e$ represent gravitational and electric force respectively)
$\implies K\frac{(Ne)^2}{r^2}=G\frac{m^2}{r^2}$
This simplifies to:
$N=\frac{m}{e}\sqrt{\frac{G}{K}}$
After plugging in the known values we obtain:
$N=6.453\times 10^6$electrons
The required fraction$=\frac{6.453\times 10^6}{7.22\times 10^{24}}=8.94\times 10^{-19}$
