How many electrons are necessary to produce a charge of - 0.90 c?

One electron has a charge of - 1.6 * 10^-19 c

x electrons have a charge of - 0.90 c

1/x = - 1.6 * 10^-19 c / - 0.9 c cancel the c's. and the - signs.

1/x = 1.6 * 10^-19 / 0.9 Cross multiply

0.9 = 1.6 * 10^-19 x Divide by 1.6 * 10^-19

0.9 / 1.6 * 10^-19 = x

x = 5.625 * 10^18 electrons.