The successive discounts 18% , 10% and 15% are equivalent to a single discount of:

37.27%

Discounts = 18% , 10% and 15% 

According to the question,

                CP            SP

18%         50           41

10%         10           9

15%         20          17

         ———————————

           10000        6237

Discount = ( 10000 - 6237 ) × \(\frac{100}{10000}\) = 37.27%