by what percentage did grant date fair value per share increase from 2013 to 2015?
during 2012 , the company granted selected employees an aggregate of 139 thousand rsus with internal performance measures and , separately , certain market thresholds . these awards vested in january 2015 . the terms of the grants specified that to the extent certain performance goals , comprised of internal measures and , separately , market thresholds were achieved , the rsus would vest ; if performance goals were surpassed , up to 175% ( 175 % ) of the target awards would be distributed ; and if performance goals were not met , the awards would be forfeited . in january 2015 , an additional 93 thousand rsus were granted and distributed because performance thresholds were exceeded . in 2015 , 2014 and 2013 , the company granted rsus , both with and without performance conditions , to certain employees under the 2007 plan . the rsus without performance conditions vest ratably over the three- year service period beginning january 1 of the year of the grant and the rsus with performance conditions vest ratably over the three-year performance period beginning january 1 of the year of the grant ( the 201cperformance period 201d ) . distribution of the performance shares is contingent upon the achievement of internal performance measures and , separately , certain market thresholds over the performance period . during 2015 , 2014 and 2013 , the company granted rsus to non-employee directors under the 2007 plan . the rsus vested on the date of grant ; however , distribution of the shares will be made within 30 days of the earlier of : ( i ) 15 months after grant date , subject to any deferral election by the director ; or ( ii ) the participant 2019s separation from service . because these rsus vested on the grant date , the total grant date fair value was recorded in operation and maintenance expense included in the expense table above on the grant date . rsus generally vest over periods ranging from one to three years . rsus granted with service-only conditions and those with internal performance measures are valued at the market value of the closing price of the company 2019s common stock on the date of grant . rsus granted with market conditions are valued using a monte carlo model . expected volatility is based on historical volatilities of traded common stock of the company and comparative companies using daily stock prices over the past three years . the expected term is three years and the risk-free interest rate is based on the three-year u.s . treasury rate in effect as of the measurement date . the following table presents the weighted-average assumptions used in the monte carlo simulation and the weighted-average grant date fair values of rsus granted for the years ended december 31: .

|  | 2015 | 2014 | 2013 |
| :--- | :--- | :--- | :--- |
| Expected volatility | 14.93% | 17.78% | 19.37% |
| Risk-free interest rate | 1.07% | 0.75% | 0.40% |
| Expected life (years) | 3.0 | 3.0 | 3.0 |
| Grant date fair value per share | $62.10 | $45.45 | $40.13 |

the grant date fair value of restricted stock awards that vest ratably and have market and/or performance and service conditions are amortized through expense over the requisite service period using the graded-vesting method . rsus that have no performance conditions are amortized through expense over the requisite service period using the straight-line method and are included in operations expense in the accompanying consolidated statements of operations . as of december 31 , 2015 , $ 4 of total unrecognized compensation cost related to the nonvested restricted stock units is expected to be recognized over the weighted-average remaining life of 1.4 years . the total grant date fair value of rsus vested was $ 12 , $ 11 and $ 9 for the years ended december 31 , 2015 , 2014 and 2013. .

grant_date_fair_value_2015 = 62.10
grant_date_fair_value_2014 = 45.45
grant_date_fair_value_2013 = 40.13
increase = grant_date_fair_value_2015 - grant_date_fair_value_2013
percent_increase = increase / grant_date_fair_value_2013
answer = percent_increase * 100