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.
