Solution 1

Let the cost price of 1616 articles be Rs.100Rs.100
Gain =35%=35%
Selling price of 1515 articles(as one is free) =100×100+35100=Rs.135=100×100+35100=Rs.135

Marked price of 1515 articles × (100−4)100=Rs.135(100−4)100=Rs.135

Marked price of 1515 articles =135×100(100−4)=140.625=135×100(100−4)=140.625

Cost price of 1515 articles =100×1516=93.75=100×1516=93.75

Required increment = (140.625−93.75)×10093.75=50%(140.625−93.75)×10093.75=50%

Marked price is 50%50% more than cost price.

Solution 2

Let the marked price of 1616 articles be Rs.100Rs.100
Then the selling price of 1616 articles =Rs.96=Rs.96 (as there is a 4%4% discount on the marked price)

Selling price of 1515 articles =96×1516=Rs.90=96×1516=Rs.90

11 article is free for buying EVERY 1515 articles.
i.e., the gain needs to be calculated by comparing the cost price of 1616 articles and selling price of 1515 articles

Cost price of 1616 articles × 135100135100 = Selling price of 1515 articles

Cost price of 1616 articles = Selling price of 1515articles ×100135=90×100135=2003×100135=90×100135=2003

Required percentage 
=(100−2003)×1002003=(32−1)100=50%=(100−2003)×1002003=(32−1)100=50%

Marked price is 50%50% more than cost price.

...Ans: =>>> 50%
I hope it's HELP U