#### Answer

$N = 8.4\times 10^{21}$

#### Work Step by Step

We can find the total charge delivered in 30 minutes:
$Q = (-0.75~C/s)(30~min)(60~s/min) = -1350~C$
We can find the number of electrons:
$N = \frac{-1350~C}{-1.6\times 10^{-19}~C} = 8.4\times 10^{21}$