$\textrm{Let’s assume cost price is 100 rs.}$
$\textrm{First discount is 10%,$\implies$ $\frac{100*10}{100}=10rs.$}$
$\textrm{so now new cost price is :100-10=90rs}$
$\textrm{now 20% discount on 90rs:$\implies$ $\frac{90*20}{100}=18rs.$}$
$\textrm{new cost price is:$90-18=72rs$}$
$\therefore$ $\textrm{Total discount is (10+18)%=28%}$