[Igo]

C3H6 + O2 > CO2 + H2O

and

H2O + Na > NaOH + H2

 

[Arkcon]

Its not too hard, really.  You just need the same amounts of each atom on either side.  What you're looking for is the least common denominator -- you didn't think grammar school math would be coming in handy this late in the game, didja?  So just keep multiplying until it works.  If a reaction can be balanced, eventually it will be by this method.

[JustinCh3m]

with combustion reactions such as this:
C3H6 + O2 > CO2 + H2O

1. balance C's first
2. then H's

why?  they're only in "one place" on each side.

3. Now make the oxygens work.  hint, you may have to use fractions and then multiply the entire balanced eq'n through, such that you end up with ALL whole number coefficients.

[ATMyller]

There is a way to solve them using pure mathmetics, and this applies to reactions with very large amount of reagents. Usually simple reactions are faster to solve using the method JustinCh3m described.

for each substance give and multiplier e.g. a, b, c and d:

a(C3H6) + b(O2) > c(CO2) + d(H2O)

and for each element build and equation like this:

for C: 3a + 0b = 1c + 0d
for H: 6a + 0b = 0c + 2d
for O: 0a + 2b = 2c + 1d

You will get a set of simultaneous equations. yo can solve the multipliers a to d by means of Gauss-jordan elimination, a graphing calculator or solve them in your head using substitution or elimination method.
if you have results with 1/2 or 1/3 multipliers you need to multipy all of a to d with 2 or 3, respectively until all multipliers are integers.

[Borek]

More details on algebraic method (with examples) here:

http://www.chembuddy.com/?left=balancing-stoichiometry&right=algebraic-method