A man buys lemons at 6 for Rs 10 and sells at 8 for Rs 15. Find his gain per cent.

Cost price of six lemons = Rs. 10
Cost price of one lemon = $\mathrm{Rs}.\frac{10}{6}$
Cost price of eight lemons = $8\times \frac{10}{6}=\mathrm{Rs}.\frac{80}{6}=\mathrm{Rs}.13.33$
Selling price of eight lemons = Rs. 15

$$\begin{gathered} \mathrm{Gain\; =\; S}P-\; CP \\ =\mathrm{Rs}.(15-13.33) \\ =\mathrm{Rs}.1.67 \end{gathered}$$

Gain% = $\left(\frac{\mathrm{Gain}}{C.P.}\times 100\right)$%
$=\left(\frac{1.67}{13.33}\times 100\right)$%
$=12.5$%