By selling 80 ball pens for Rs. 140 a retailer losses 30%. How many ball pens should he sell for Rs. 104 so as to make a profit of 30% ?

$$\eqalign{ & 100x \times \frac{{100 - 30}}{{100}} = 140 \cr & \Rightarrow 70x = 140 \cr & \Rightarrow x = 2 \cr & \therefore {\text{CP}} = 100x \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 100 \times 2 \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = {\text{Rs}}{\text{.}}\,200 \cr & {\text{SP}} = 200 \times \frac{{\left( {100 + 30} \right)}}{{100}} \cr & \,\,\,\,\,\,\,\,\, = 260 \cr & {\text{Now,}} \cr & {\text{Rs}}{\text{.}}\,260 = 80\,{\text{ball}}\,{\text{pens}} \cr & \therefore {\text{Rs}}{\text{.}}\,104 = \frac{{80}}{{260}} \times 104 \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 32\,{\text{ball}}\,{\text{pens}} \cr} $$