The microwave has 60% efficiency, this means that the power delivered (P=900 W) is 60% of the original power of the source, while the other 40% is dissipated.
We can write it this way:
[tex]P= 0.60 P_{i}[/tex]
where [tex]P_i[/tex] is the power of the source, which we can find by re-arranging the equation:
[tex]P_i = \frac{P}{0.60}= \frac{900 W}{0.60}=1500 W [/tex]
