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: .

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

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

Question: what are the expected annual cash interest costs for the 6.625% ( 6.625 % ) senior notes?
Answer: 28125000

Question: what is the percentage change in interest expense from 2005 to 2006?
Answer: 15.5%

Question: what is the average balance of letters of credit outstanding as of december 31 , 2007 and 2006 , in millions?
Answer: 46.7

Question: what is the percentage change in interest expense from 2006 to 2007?
Answer: 11.2%