By selling 125 cassettes a man gains an amount equal to the selling price of 5 cassettes. Find he gain percent

Let the SP of one cassette be Rs. x.

Then,

SP of 5 cassettes(Profit) = Rs. 5x 

SP of 125 cassettes(Total SP) = Rs. 125x

CP of 125 cassettes = SP - Profit = Rs. (125x - 5x) = Rs. 120x

Therefore, Gain % = (gain/CP) * 100 = (5x/120x) * 100 = 4.16% (approx.)