Question

An asteroid has acquired a net negative charge of 126C from being bombarded by the solar wind over the years, and is currently in equilibrium whereby it expels electrons at the same rate as it acquires them. How many more electrons does it have than protons?

Answer 1

The net charge of the asteroid is
$Q=-126 C$
with a negative sign since it is a net negative charge. 

The charge of one electron is
$e=-1.6 \cdot 10^{-19} C$

In order to find what is the excess of electrons on the asteroid, we must simply divide the net charge in excess on the asteroid by the charge of a single electron, and we find:
$N= \frac{Q}{e}= \frac{-126 C}{-1.6 \cdot 10^{-19} C} =7.87 \cdot 10^{20}$