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Felecityyy
The answer would be 5.

Equation = C3 H8 + O2 → Co2 + H2O
Balanced equation = C3H8 + O2 → 3CO2 + 4H2O
In the first part, left side, has 2 oxygens, and the next one has 10. So in order to turn 2 into 10, add the coefficient 5.