By selling 130 cassettes, a man gains an amount equal to the selling price of 5 cassettes.

It is given that,

Gain = SP of 5 cassettes                 .....(1)

Gain = SP of 130 cassettes − CP of 130 cassettes

⇒ SP of 5 cassettes = SP of 130 cassettes − CP of 130 cassettes               [From (1)]

⇒ CP of 130 cassettes = SP of 125 cassettes          .....(2)

Let the CP of 1 cassettte be ₹x.

∴ CP of 125 cassettes = ₹125x

CP of 130 cassettes = ₹130x

SP of 125 cassettes = CP of 130 cassettes                    [From (2)]

⇒ SP of 125 cassettes = ₹130x

Now, gain $\%=\frac{\mathrm{SP}-\mathrm{CP}}{\mathrm{CP}} \times 100 \%=\frac{(130 x-125 x)}{125 x} \times 100 \%=\frac{5 x}{125 x} \times 100 \%=4 \%$

Thus, the gain percent is 4%.