Successive discounts of 10%, 20% will be equivalent to a single discount is
successive discount of x% and y% = $$ (x + y - \frac{xy}{100})$$%
here x% = 10%
y% = 20%
required discount = $$ (20 + 10 - \frac{20 \times 10}{100}) $$%
              = 28%
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