Oranges are bought at 6 for ₹ 20 and sold at 4 for ₹ 18.

LCM of 6 and $4=12$

Let the number of oranges bought be $12 .$

CP of 6 oranges $=₹ 20$

So, CP of 1 orange $=\frac{20}{6}=₹ \frac{10}{3}$

CP of 12 orange $=12 \times \frac{10}{3}=₹ 40$

SP of 4 oranges $=₹ 18$

SP of 1 orange $=\frac{18}{4}=₹ \frac{9}{2}$

SP of 12 oranges $=12 \times \frac{9}{2}=₹ 54$

Here, SP of 12 oranges $>\mathrm{CP}$ of 12 oranges.

Profit $=\mathrm{SP}-\mathrm{CP}=₹ 54-₹ 40=₹ 14$

$\therefore$ Profit $\%=\frac{\text { Profit }}{\mathrm{CP}} \times 100 \%=\frac{14}{40} \times 100 \%=35 \%$