The total time the lamp was on is
[tex]t=4 h \cdot 30 d=120 h[/tex]
Since the power is P=125 W, the total energy consumed by the lamp is
[tex]E=Pt=(125 W)(120 h)=15000 Wh=15 kWh[/tex]
Each 1 kWh costs 10 cents, so the total cost will be
[tex]15 kWh \cdot 10 \frac{cent}{kWh}=150 cents=1.50 [/tex] dollars.
