Source: High school physics (Chinese)
Problem
In a gas discharge tube, when the voltage between the two electrodes is sufficiently high, a large number of gas molecules are ionized into electrons and positive ions, making the gas conductive. In a hydrogen gas discharge tube, $3.1 \times 10^{18}$ electrons and $1.1 \times 10^{18}$ hydrogen ions pass through a certain cross-section of the tube per second.
Electrons and positive ions move in opposite directions, but their contributions to the conventional current add together (negative charges moving one way is equivalent to positive charges moving the other way).
The total charge crossing the section per second is:
$$I = (N_e + N_+) e$$where $e = 1.6 \times 10^{-19}$ C is the elementary charge.
Substituting:
$$I = (3.1 \times 10^{18} + 1.1 \times 10^{18}) \times 1.6 \times 10^{-19} \text{ A}$$ $$I = 4.2 \times 10^{18} \times 1.6 \times 10^{-19} \text{ A} \approx 0.67 \text{ A}$$The direction of the conventional current is the direction in which the positive hydrogen ions move (from the anode to the cathode inside the tube).