An internal combustion engine does 356 kJ of useful work using 946 kJ of thermal energy from the gasoline consumed in the engine. What is the efficiency of this engine?

The efficiency of this engine is 37.63 %.

Explanation:

Given;

useful output work done by the combustion engine, = 356 kJ

input work, = 946 kJ

The efficiency of the combustion engine is calculated as;

Therefore, the efficiency of this engine is 37.63 %.

The green that grows on the copper penny, and the rust that forms
on the iron nail, are chemical changes.

Boiling salt water away, extracting iron filings from a sand mixture with
a magnet, and crushing a rock with a hammer, are physical changes.

Extracting iron filings from a sand mixture with a magnet isn't a change
at all.  That's just separating things that started out mixed.