Answer
$10^{18} $
Work Step by Step
John = 90 kg
Mary = 45 kg
d = 30 m
The net charge that John carries is
$q_{John} = (0.0001 \%) \frac{mN_AZe}{M}$
$q_{John} = (0.0001) \frac{90kg(6.02 \times 10^{23}/mol) (18)(1.6 \times 10^{-19} C)}{0.018 kg/mol}$
$q_{John} = 866800 C$
Mary has half of the weight of John, this makes
$q_{Mary} = \frac{1}{2} q_{John}$
$q_{Mary} = 433440 C$
And the force of these two is
$F = \frac{kq_{John}q_{Mary}}{d^2}$
$F = \frac{(8.99 \times 10^9 N . m^2 / C^2)(866800 C)(433440 C)}{(30m)^2} $
$F = 3.8 \times 10^{18} N$
$10^{18} $ is the order of magnitude of the electrostatic force.
