Cost Price = ₹ 100
    Marked Price = ₹ 100 + 25% of ₹ 100 = ₹ 125

    To get 15 % Profit the selling Price will be ₹ 115

    So discount percentage will be = ((125 - 115)/125) * 100
=   8 %

[ Note: While calculating profit for the seller cost price is taken in the denominator and while
calculating discount for the customer selling price is taken in the denominator. ]