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(d) 25%

L. C.M of 5 and $6=(5 \times 1 \times 6)=30$

Let 30 be the number of oranges bought.

$\mathrm{CP}$ of 5 oranges $=\mathrm{Rs} 10$

CP of 1 oranges $=$ Rs $\left(\frac{10}{5}\right)$

= Rs 2

$\therefore \mathrm{CP}$ of 30 oranges $=\mathrm{Rs}(2 \times 30)$

= Rs 60

SP of 6 oranges $=$ Rs 15

SP of 1 oranges $=\operatorname{Rs}\left(\frac{15}{6}\right)$

$\therefore \mathrm{SP}$ of 30 oranges $=\mathrm{Rs}\left(\frac{15}{6} \times 30\right)$

= Rs 75

Now, gain $=\mathrm{Rs}(75-60)$

= Rs 15

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

$=\left(\frac{15}{60} \times 100\right) \%$

= 25%