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% ( 14.93 % )', '17.78% ( 17.78 % )', '19.37% ( 19.37 % )'], ['risk-free interest rate', '1.07% ( 1.07 % )', '0.75% ( 0.75 % )', '0.40% ( 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. .

Question: by what percentage did grant date fair value per share increase from 2013 to 2015?
Answer: 54.7%