Q.Find the single discount equivalent to two successive discounts of 20% and 10%.

Successive discounts multiply the retained fractions; the single equivalent is what is lost overall.

Take a convenient list price of ₹100. After a $20\%$ discount, ₹80 remains. A further $10\%$ is taken on this ₹80, i.e. ₹8, leaving ₹72. So ₹28 out of ₹100 is the total discount — a single discount of $28\%$.

Using the formula directly:

$D=d_1+d_2-\frac{d_1 d_2}{100}=20+10-\frac{20\times 10}{100}=30-2=28\%.$ …

Simply adding the discounts to get $30\%$. Successive discounts are applied one after another, so the equivalent single discount …