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Effective discount from the combined factor
The true single rate that matches the chain. For 0.20 and 0.10 it is 0.28, not the 0.30 of a plain sum.
The Formula
Effective = 1 - (1 - a)(1 - b) = a + b - a x b
The true single rate that matches the chain. For 0.20 and 0.10 it is 0.28, not the 0.30 of a plain sum.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| a | First discount as a decimal | decimal | 0.20 in the worked example. |
| b | Second discount as a decimal | decimal | 0.10 in the worked example. |
| d_eff | Effective discount | decimal | The single rate equivalent to the chain: 0.28 here, applied once to the original price. |
What Each Symbol Means
adecimal
First discount as a decimal
0.20 in the worked example.
bdecimal
Second discount as a decimal
0.10 in the worked example.
d_effdecimal
Effective discount
The single rate equivalent to the chain: 0.28 here, applied once to the original price.
See It Worked With Real Numbers
A formula on its own does not tell you which number goes where. The how to calculate stacked discounts page applies this equation to a concrete case, shows every intermediate step, and names the mistakes that produce a wrong answer even when the algebra is right.
Where This Formula Appears
This equation is the working core of How To Calculate Stacked Discounts, filed under Everyday Life. Every number the guide produces comes from substituting into this one line.