Question

A microwave oven running at 60 % efficiency delivers 900 w of energy per second to the interior. find the power drawn from the source.

Answer 1

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:
$P= 0.60 P_{i}$
where $P_i$ is the power of the source, which we can find by re-arranging the equation:
$P_i = \frac{P}{0.60}= \frac{900 W}{0.60}=1500 W$