aeronautics our aeronautics business segment is engaged in the research , design , development , manufacture , integration , sustainment , support and upgrade of advanced military aircraft , including combat and air mobility aircraft , unmanned air vehicles and related technologies . aeronautics 2019 major programs include the f-35 lightning ii joint strike fighter , c-130 hercules , f-16 fighting falcon , c-5m super galaxy and f-22 raptor . aeronautics 2019 operating results included the following ( in millions ) : .

2015: net sales: 15570
operating profit: 1681
operating margins: -10.8
backlog at year-end: 31800
2014: net sales: 14920
operating profit: 1649
operating margins: -11.1
backlog at year-end: 27600
2013: net sales: 14123
operating profit: 1612
operating margins: -11.4
backlog at year-end: 28000

2015 compared to 2014 aeronautics 2019 net sales in 2015 increased $ 650 million , or 4% ( 4 % ) , compared to 2014 . the increase was attributable to higher net sales of approximately $ 1.4 billion for f-35 production contracts due to increased volume on aircraft production and sustainment activities ; and approximately $ 150 million for the c-5 program due to increased deliveries ( nine aircraft delivered in 2015 compared to seven delivered in 2014 ) . the increases were partially offset by lower net sales of approximately $ 350 million for the c-130 program due to fewer aircraft deliveries ( 21 aircraft delivered in 2015 , compared to 24 delivered in 2014 ) , lower sustainment activities and aircraft contract mix ; approximately $ 200 million due to decreased volume and lower risk retirements on various programs ; approximately $ 195 million for the f-16 program due to fewer deliveries ( 11 aircraft delivered in 2015 , compared to 17 delivered in 2014 ) ; and approximately $ 190 million for the f-22 program as a result of decreased sustainment activities . aeronautics 2019 operating profit in 2015 increased $ 32 million , or 2% ( 2 % ) , compared to 2014 . operating profit increased by approximately $ 240 million for f-35 production contracts due to increased volume and risk retirements ; and approximately $ 40 million for the c-5 program due to increased risk retirements . these increases were offset by lower operating profit of approximately $ 90 million for the f-22 program due to lower risk retirements ; approximately $ 70 million for the c-130 program as a result of the reasons stated above for lower net sales ; and approximately $ 80 million due to decreased volume and risk retirements on various programs . adjustments not related to volume , including net profit booking rate adjustments and other matters , were approximately $ 100 million higher in 2015 compared to 2014 . 2014 compared to 2013 aeronautics 2019 net sales increased $ 797 million , or 6% ( 6 % ) , in 2014 as compared to 2013 . the increase was primarily attributable to higher net sales of approximately $ 790 million for f-35 production contracts due to increased volume and sustainment activities ; about $ 55 million for the f-16 program due to increased deliveries ( 17 aircraft delivered in 2014 compared to 13 delivered in 2013 ) partially offset by contract mix ; and approximately $ 45 million for the f-22 program due to increased risk retirements . the increases were partially offset by lower net sales of approximately $ 55 million for the f-35 development contract due to decreased volume , partially offset by the absence in 2014 of the downward revision to the profit booking rate that occurred in 2013 ; and about $ 40 million for the c-130 program due to fewer deliveries ( 24 aircraft delivered in 2014 compared to 25 delivered in 2013 ) and decreased sustainment activities , partially offset by contract mix . aeronautics 2019 operating profit increased $ 37 million , or 2% ( 2 % ) , in 2014 as compared to 2013 . the increase was primarily attributable to higher operating profit of approximately $ 85 million for the f-35 development contract due to the absence in 2014 of the downward revision to the profit booking rate that occurred in 2013 ; about $ 75 million for the f-22 program due to increased risk retirements ; approximately $ 50 million for the c-130 program due to increased risk retirements and contract mix , partially offset by fewer deliveries ; and about $ 25 million for the c-5 program due to the absence in 2014 of the downward revisions to the profit booking rate that occurred in 2013 . the increases were partially offset by lower operating profit of approximately $ 130 million for the f-16 program due to decreased risk retirements , partially offset by increased deliveries ; and about $ 70 million for sustainment activities due to decreased risk retirements and volume . operating profit was comparable for f-35 production contracts as higher volume was offset by lower risk retirements . adjustments not related to volume , including net profit booking rate adjustments and other matters , were approximately $ 105 million lower for 2014 compared to 2013. .

Questions:
what was the backlog value at the end of 2015?
what was the backlog value at the end of 2014?
what is the sum of the backlog value in 2014 and 2015?
what was the backlog value at the end of 2013?
what is the sum of the backlog value for 2013, 2014 and 2015?
what is the average backlog value per year?

Answers:
31800
27600
59400
28000
87400
29133