A single discount equivalent to four successive discount of 6%, 9%, 15% and 20% is (approx):

41.8%

6% = \(\frac{3}{50}\) 

9% = \(\frac{9}{100}\)  

15% = \(\frac{3}{20}\)  

20% = \(\frac{1}{5}\)

Single discount equivalent =  \(\frac{47}{50}\) ×  \(\frac{ 91 }{100}\) × \(\frac{17}{20}\) × \(\frac{4}{5}\)

Single discount equivalent =  \(\frac{290836}{500000}\)

Single discount equivalent =  \(\frac{209164 × 100}{500000}\)  = 41.8%