Without doing any calculations, list the elements in c2h4o2 in order of decreasing mass percent composition.

In c2h4o2 there are
2 atoms of carbon
4 atoms of hydrogen
2 atoms of oxygen

Hydrogen atomic masses is 1 which was very low so 4 of them would still be the lowest mass percent. Then we have carbon and oxygen which both have 2 atoms. Oxygen atomic mass is more than carbon atomic mass, so oxygen should contribute more mass percent composition. The sequence from higher mass percent composition should be:
1. Oxygen
2. Carbon
3. Hydrogen