By selling 45 lemons, a vendor loses a sum equal to the selling price of 3 lemons.

Let Rs $x$ be the SP of one lemon.

SP of 45 lemons $=$ Rs. $45 x$

Loss $=\mathrm{SP}$ of 3 lemons $=$ Rs. $3 x$

But loss $=\mathrm{CP}-\mathrm{SP}$

$\mathrm{CP}=$ loss $+\mathrm{SP}$

$=3 x+45 x$

$=$ Rs. $48 x$

$\therefore$ Loss percentage $=\left(\frac{\text { loss }}{\mathrm{CP}} \times 100\right) \%$

$=\left(\frac{3 x}{48 x} \times 100\right) \%$

$=6 \frac{1}{4} \%$