what was the percentage growth in the weighted-average estimated fair values of stock options granted from 2009 to 2010
2006 plan prior to december 5 , 2008 became fully vested and nonforfeitable upon the closing of the acquisition . awards may be granted under the 2006 plan , as amended and restated , after december 5 , 2008 only to employees and consultants of allied waste industries , inc . and its subsidiaries who were not employed by republic services , inc . prior to such date . at december 31 , 2010 , there were approximately 15.3 million shares of common stock reserved for future grants under the 2006 plan . stock options we use a binomial option-pricing model to value our stock option grants . we recognize compensation expense on a straight-line basis over the requisite service period for each separately vesting portion of the award , or to the employee 2019s retirement eligible date , if earlier . expected volatility is based on the weighted average of the most recent one-year volatility and a historical rolling average volatility of our stock over the expected life of the option . the risk-free interest rate is based on federal reserve rates in effect for bonds with maturity dates equal to the expected term of the option . we use historical data to estimate future option exercises , forfeitures and expected life of the options . when appropriate , separate groups of employees that have similar historical exercise behavior are considered separately for valuation purposes . the weighted-average estimated fair values of stock options granted during the years ended december 31 , 2010 , 2009 and 2008 were $ 5.28 , $ 3.79 and $ 4.36 per option , respectively , which were calculated using the following weighted-average assumptions: .

|  | 2010 | 2009 | 2008 |
| :--- | :--- | :--- | :--- |
| Expected volatility | 28.6% | 28.7% | 27.3% |
| Risk-free interest rate | 2.4% | 1.4% | 1.7% |
| Dividend yield | 2.9% | 3.1% | 2.9% |
| Expected life (in years) | 4.3 | 4.2 | 4.2 |
| Contractual life (in years) | 7 | 7 | 7 |
| Expected forfeiture rate | 3.0% | 3.0% | 3.0% |

republic services , inc . notes to consolidated financial statements , continued .

fair_value_2010 = 5.28
fair_value_2009 = 3.79
fair_value_2008 = 4.36
fair_value_growth = (fair_value_2010 - fair_value_2009) / fair_value_2009
answer = fair_value_growth * 100