First of all, let's convert the life duration of the bulb from hours into seconds:
[tex]t=1500 h = 5.4 \cdot 10^6 s[/tex]
So now we can calculate the total energy consumed by the first light bulb:
[tex]E_1=Pt = (75 W)(5.4 \cdot 10^6 s)=4.05 \cdot 10^8 J[/tex]
While the total energy consumed by the second light bulb is
[tex]E_2=Pt = (18 W)(5.4 \cdot 10^6 s)=9.7 \cdot 10^7 J[/tex]
So the total energy saved by replacing the light bulb is
[tex]\Delta E=E_1 - E_2 = 4.05 \cdot 10^8 J-9.7 \cdot 10^7 J=3.08 \cdot 10^8 J[/tex]
