The equivalent single discount for two successive discounts of 15% and 10% is = ?

Equivalent to a single discount
$$\eqalign{ & = 15 + 10 - \frac{{15 \times 10}}{{100}} \cr & = 25 - 1.5 \cr & = 23.5\% \cr} $$

Alternate :
$$\eqalign{ & {\text{Market price = Rs}}{\text{.100}} \cr & {\text{Cost price}} \cr & {\text{ = 100}} \times \frac{{85}}{{100}} \times \frac{{90}}{{100}} \cr & {\text{ = Rs}}{\text{. }}76.5 \cr & {\text{Difference = 100}} - 76.5 \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 23.5 \cr & {\text{Difference}}\% \cr & = \frac{{23.5}}{{100}} \times 100 \cr & = 23.5\% \cr} $$