One Joule is the amount of energy generated by a power of one Watt in one second.
Knowing this, we can write the formula below:
[tex]E=P\cdot t[/tex]
So, for an energy of 42000 J and a power of 1200 W, the time required is:
[tex]\begin{gathered} 42000=1200\cdot t \\ t=\frac{42000}{1200} \\ t=35\text{ seconds} \end{gathered}[/tex]
The number of significant figures in the number 35 is two.
