Manish purchased 25 kg of rice @ Rs. 32 per kg and 15 kg of rice @ Rs. 36 per kg. He mixed the two varieties of rice and sold it @ Rs. 40.20 per kg. What is the percent profit earned ?

$$\eqalign{ & {\text{C}}{\text{.P}}{\text{. of 40 kg rice}} \cr & = {\text{Rs}}.\left( {25 \times 32 + 15 \times 36} \right) \cr & = {\text{Rs}}.\left( {800 + 540} \right) \cr & = {\text{Rs}}.1340. \cr & {\text{S}}{\text{.P}}{\text{. of 40 kg rice}} \cr & = {\text{Rs}}.\left( {40 \times 40.20} \right) \cr & = {\text{Rs}}{\text{. }}1608. \cr & {\text{Profit}} = {\text{Rs}}.\left( {1608 - 1340} \right) \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = {\text{Rs}}{\text{. }}268. \cr & \therefore {\text{Profit }}\% = \left( {\frac{{268}}{{1340}} \times 100} \right)\% \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 20\% \cr} $$