Read the following texts and table with financial data from an S&P 500 earnings report carefully.Based on the question-answer history (if provided), answer the last question. The answer may require mathematical calculation based on the data provided.

Text: 2 0 0 8 a n n u a l r e p o r t stock performance graph the following graph sets forth the performance of our series a common , series b common stock , and series c common stock for the period september 18 , 2008 through december 31 , 2008 as compared with the performance of the standard and poor 2019s 500 index and a peer group index which consists of the walt disney company , time warner inc. , cbs corporation class b common stock , viacom , inc . class b common stock , news corporation class a common stock , and scripps network interactive , inc . the graph assumes $ 100 originally invested on september 18 , 2006 and that all subsequent dividends were reinvested in additional shares . september 18 , september 30 , december 31 , 2008 2008 2008 . <table class='wikitable'><tr><td>1</td><td>-</td><td>september 18 2008</td><td>september 30 2008</td><td>december 31 2008</td></tr><tr><td>2</td><td>disca</td><td>$ 100.00</td><td>$ 103.19</td><td>$ 102.53</td></tr><tr><td>3</td><td>discb</td><td>$ 100.00</td><td>$ 105.54</td><td>$ 78.53</td></tr><tr><td>4</td><td>disck</td><td>$ 100.00</td><td>$ 88.50</td><td>$ 83.69</td></tr><tr><td>5</td><td>s&p 500</td><td>$ 100.00</td><td>$ 96.54</td><td>$ 74.86</td></tr><tr><td>6</td><td>peer group</td><td>$ 100.00</td><td>$ 92.67</td><td>$ 68.79</td></tr></table> s&p 500 peer group .
Question: from september 30 to december 31 of 2008, what was the decline in the performance price of the s&p 500?
Answer: 21.68
Question: and what was that price in december 31?
Answer: 74.86
Question: what is, then, that decline as a portion of this december amount?

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