18% discount and then 20% discount in succession is equivalent to total discount of

Successive discount can be given by $=x+y+\frac{xy}{100}$

$\begin{array}{l}=-10-20+\frac{(-10\times -20)}{100}\\ =-30+2=28\mathrm{\%}\end{array}$

Hence, the successive dicount in equal to 28%