The power P is defined as the energy E per unit of time t:
[tex]P= \frac{E}{t} [/tex]
In our problem, the power is P=25 W and the total time is t=8.0h, so we can calculate the total energy used during this time:
[tex]E=Pt=(25 W)(8.0 h)=200 Wh=0.2 kWh[/tex]
The energy costs 8€ per each kwh, so we can write the following proportion:
[tex]8€: 1kwh = x:0.2 kWh[/tex]
where x is the price corresponding to 0.2 kWh. Solving the proportion we find x:
[tex]x= \frac{(8€)(0.2 kWh)}{1kWh}=1.6 [/tex]€
