(b) Net current $i = i_{+} + i_{-} = \frac{(n_{+})(q_{+})}{t} + \frac{(n_{-})(q_{-})}{t}$
$\Rightarrow i = \frac{(n_{+})}{t} \times e + \frac{(n_{-})}{t} \times e$
$= 2.9 \times 10^{18} \times 1.6 \times 10^{- 19} + 1.2 \times 10^{18} \times 1.6 \times 10^{- 19}$
$\Rightarrow i = 0.66\ A$