Answer: Option C
Let the cost price of goods = Rs. 100
Market price of goods
$$\eqalign{
& {\text{ = 120}}\% {\text{ of 100 }} \cr
& {\text{ = }}\frac{{120}}{{100}} \times {\text{100}} \cr
& {\text{ = Rs}}{\text{. 120}} \cr} $$
After discount selling price of goods = 70% of 120 = Rs. 84
$$\eqalign{
& {\text{Loss = 100}} - 84 = {\text{Rs}}{\text{.16}} \cr
& {\text{Loss }}\% {\text{ = }}\frac{{16}}{{100}} \times 100 = 16\% \cr} $$
Alternate :
$$\eqalign{
& {\text{Formula = }}x \pm y \pm {\text{ }}\frac{{x \times y}}{{100}} \cr
& {\text{Loss}}\% {\text{ = 20}} - {\text{30}} - \frac{{20 \times 30}}{{100}} \cr
& \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = - 10 - 6 \cr
& \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = - 16\% {\text{ }}({\text{Loss)}} \cr} $$