A household refrigerator's rated power, 100W, if it is used for 10 hours everyday, calculate the cost of running it for 30 days when one unit of electricity is 30. Calculate the total cost.

The power of the refrigerator is $200 \text{ W}$, and it runs for $10 \text{ hours}$ each day.

$$E_{day} = \text{Power} \times \text{Time per day}$$ $$E_{day} = 200 \text{ W} \times 10 \text{ h} = 2000 \text{ Wh}$$

Since $1 \text{ kWh} = 1000 \text{ Wh}$:

$$E_{day, kWh} = \frac{2000 \text{ Wh}}{1000 \text{ Wh/kWh}} = 2 \text{ kWh}$$

The refrigerator runs for $20 \text{ days}$.

$$E_{total} = E_{day, kWh} \times \text{Number of days}$$ $$E_{total} = 2 \text{ kWh/day} \times 20 \text{ days} = 40 \text{ kWh}$$

One unit of electricity (1 kWh) costs $₦2$.

$$\text{Total Cost} = E_{total} \times \text{Cost per kWh}$$ $$\text{Total Cost} = 40 \text{ kWh} \times ₦2/\text{kWh} = ₦80$$

The total cost of running the refrigerator for 20 days is $\boxed{₦80}$.