Calculating the cost!!

Energy in a circuit which has a device with a potential drop of $V$ and a current of $I$ passing through it for time $t$ , is given by

$E = VIt$

For our case, this energy is contributed by two devices, i.e. a $250V$ , $1A$ toaster and a $300V$ , $1.5A$ heater. So, the total energy can be given by:

$E = (250)(1)(30)(60) + (300)(1.5)(50)(60) = 54 \times {10}^{6} J$

We know that $3.6 \times {10}^{6} J = 1kWh$ , and so, our energy in $kWh$ is $E = \dfrac {54 \times {10}^{6}} {3.6 \times {10}^{6}} = 15kWh$ .

So, the cost of electrical energy used is $15 \times 4 = 60 = n$ and thus ${n}^{3} = 216000$ .