I would like to aplly successively to reudction percentage to a price and I want to find the "global" reduction.
Apply a reduction of 19.6% and a 20% is different to apply a reduction of 39.6%.
So, i try to calculate the cumulative coefficient reduction $( 1 - 0.196) * ( 1 - 0.2 ) = 0.804 * 0.8 = 0.6432$
I thought it was a reduction of $(1 - 0.6432) * 100 = 35.68%$ ... but I think I do a mistake because
$(100 / 1.196) / 1.2 = 69.68$ And $100/1.3568 = 73.70$
So... what is a good way to calculate the global percentage of 2 successives reductions ?
Your original calculation is correct: taking off $19.6$% and $20$% in succession is equivalent to taking off $35.68$% all at once. The other calculation doesn’t appear to have anything to do with the problem; there is certainly no reason to expect those two quantities to be equal (even after you correct $1.96$ to $1.196$).
For example, if you start with $100$, taking off $19.6$% leaves you with $80.4$; $20$% of $80.4$ is $16.08$, and $80.4-16.08=64.32$, which is indeed what’s left when you remove $35.68$% of $100$.