The total amount of energy can be calculated as the product of the power and the time the iron worked.
So the energy will be "900\\,\\mathrm{W}\\cdot 4\\,\\mathrm{h}\\cdot5 = 18000\\,\\mathrm{W\\cdot h} = 18\\,\\mathrm{kW h}" .
Therefore, the cost will be "18\\,\\mathrm{kW h} \\cdot 30\\,\\mathrm{k\/kWh} = 540\\,\\mathrm{k}."
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