Step 1
The conservation of matter or mass is applied here. The same atoms must appear on both sides of the reaction.
The same number of atoms must appear on both sides of the reaction too.
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Step 2
The reaction:
2 C2H6 + 7 O2 => 4 CO2 + 6 H2O (this is a combustion reaction)
The numbers next to the molecules are coefficients.
According to the question, the answer is:
2 C2H6
7 O2
4 CO2
6 H2O
