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Python 3, 86 * 0.9 = 77.4 bytes



s=input()
N=int(s.split('H')[1])
print("2%s + %dO2 &gt; %dH20 + %dCO2"%(s,N*1.5-2,N,N-2))


Extracts the number of H's rather than the number of C's from the input. This avoid special-casing CH4 and simplifies the output expressions in terms of N=2n+2.

The output has parameters plugged in via string formatting. The first summand is just the input string, and the rest have calculated numbers plugged in. Note that N*1.5-2 (same as N*3/2-2) gives a float, but the string formatting converts it to an int.