Correct option is A. 420
It is given that the $$\mathrm{S.P}$$ is same for both the fans.
Let $$\mathrm{C.P} $$ of the first fan be $$\mathrm{Rs.}\space x$$
$$\therefore \mathrm{C.P}$$ of the second fan $$=\mathrm{Rs.}\space (3605-x)$$
Profit on the first fan $$=15 \%$$
Loss on the second fan $$=9 \%$$
For the first fan,
$$\mathrm{S.P}=\mathrm{C.P}\left(\frac{100+\mathrm{Gain} \%}{100}\right)$$
$$=x\left(\frac{115}{100}\right)$$
$$=\frac{23 x}{20}$$
For the second fan, $$\mathrm{S.P}=\mathrm{C.P}\left(\frac{100-\mathrm{Loss} \%}{100}\right)$$
$$=(3605-x)\left(\frac{91}{100}\right)$$
Since $$\mathrm{S.P}$$ of both the fans is the same,
$$\therefore \frac{23 x}{20}=(3605-x)\left(\frac{91}{100}\right)$$
$$\Rightarrow 2300 x=91(72100-20 x)$$
$$\Rightarrow 2300 x=6561100-1820 x$$
$$\Rightarrow 4120 x=6561100$$
$$\Rightarrow x=\mathrm{Rs.} \space1592.50$$
Thus, $$\mathrm{C.P}$$ of the first fan is $$\mathrm{Rs.} \space1592.50$$ .
$$\mathrm{C.P}$$ of the second fan $$=\mathrm{Rs.}\space (3605-1592.50)=\mathrm{Rs.} \space2012.50$$
difference between the cost price of first and second fan $$=2012.50-1592.50=\text{Rs}\,420$$