page 59 of 94 notes to consolidated financial statements ball corporation and subsidiaries 13 . debt and interest costs ( continued ) long-term debt obligations outstanding at december 31 , 2007 , have maturities of $ 127.1 million , $ 160 million , $ 388.4 million , $ 625.1 million and $ 550.3 million for the years ending december 31 , 2008 through 2012 , respectively , and $ 456.1 million thereafter . ball provides letters of credit in the ordinary course of business to secure liabilities recorded in connection with industrial development revenue bonds and certain self-insurance arrangements . letters of credit outstanding at december 31 , 2007 and 2006 , were $ 41 million and $ 52.4 million , respectively . the notes payable and senior credit facilities are guaranteed on a full , unconditional and joint and several basis by certain of the company 2019s domestic wholly owned subsidiaries . certain foreign denominated tranches of the senior credit facilities are similarly guaranteed by certain of the company 2019s wholly owned foreign subsidiaries . note 22 contains further details as well as condensed , consolidating financial information for the company , segregating the guarantor subsidiaries and non-guarantor subsidiaries . the company was not in default of any loan agreement at december 31 , 2007 , and has met all debt payment obligations . the u.s . note agreements , bank credit agreement and industrial development revenue bond agreements contain certain restrictions relating to dividend payments , share repurchases , investments , financial ratios , guarantees and the incurrence of additional indebtedness . on march 27 , 2006 , ball expanded its senior secured credit facilities with the addition of a $ 500 million term d loan facility due in installments through october 2011 . also on march 27 , 2006 , ball issued at a price of 99.799 percent $ 450 million of 6.625% ( 6.625 % ) senior notes ( effective yield to maturity of 6.65 percent ) due in march 2018 . the proceeds from these financings were used to refinance existing u.s . can debt with ball corporation debt at lower interest rates , acquire certain north american plastic container net assets from alcan and reduce seasonal working capital debt . ( see note 3 for further details of the acquisitions. ) on october 13 , 2005 , ball refinanced its senior secured credit facilities to extend debt maturities at lower interest rate spreads and provide the company with additional borrowing capacity for future growth . during the third and fourth quarters of 2005 , ball redeemed its 7.75% ( 7.75 % ) senior notes due in august 2006 . the refinancing and senior note redemptions resulted in a debt refinancing charge of $ 19.3 million ( $ 12.3 million after tax ) for the related call premium and unamortized debt issuance costs . a summary of total interest cost paid and accrued follows: .

2007: interest costs before refinancing costs: 155.8
debt refinancing costs: 2013
total interest costs: 155.8
amounts capitalized: -6.4
interest expense: 149.4
interest paid during the year ( a ): 153.9
2006: interest costs before refinancing costs: 142.5
debt refinancing costs: 2013
total interest costs: 142.5
amounts capitalized: -8.1
interest expense: 134.4
interest paid during the year ( a ): 125.4
2005: interest costs before refinancing costs: 102.4
debt refinancing costs: 19.3
total interest costs: 121.7
amounts capitalized: -5.3
interest expense: 116.4
interest paid during the year ( a ): 138.5

( a ) includes $ 6.6 million paid in 2005 in connection with the redemption of the company 2019s senior and senior subordinated notes. .

Questions:
what was the total balance of letters of credit outstanding for the years of 2006 and 2007, combined?
and what was the average balance between those two years?
and in that year of 2006, what was the interest expense?
what was it in 2005?
what was, then, the change over the year?
and what is this change as a percentage of that 2005 interest expense?

Answers:
93.4
46.7
134.4
116.4
18.0
15.5%