A radio is sold for Rs 3120 at a loss of 4%.

Solution:

Let the original price be $x$.

SP = Rs 3120

Now, SP = CP - loss

$\Rightarrow 3120=x-\frac{4}{\frac{100}{x}}$

$\Rightarrow 3120=x-\frac{x}{25}$

$\Rightarrow 3120=\frac{24 x}{25}$

$\Rightarrow \frac{3120 \times 25}{24}=x$

$\Rightarrow x=3250$

So, the cost price is Rs 3250 .

If it is sold for Rs 3445 , then its a gain because $S P>C P$.

Now, gain = SP - CP

$=\operatorname{Rs}(3445-3250)$

= Rs 195

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

$=\left(\frac{195}{3250} \times 100\right) \%$

$=6 \%$

Hence, gain percent $=6 \%$