if the same amount was spent monthly for 24 months purchasing $ 2.5 billion of common stock , what was the monthly average spent be , in billions?
table of contents valero energy corporation notes to consolidated financial statements ( continued ) 11 . equity share activity activity in the number of shares of common stock and treasury stock was as follows ( in millions ) : common treasury .

|  | CommonStock | TreasuryStock |
| :--- | :--- | :--- |
| Balance as of December 31, 2015 | 673 | (200) |
| Transactions in connection withstock-based compensation plans | — | 1 |
| Stock purchases under purchase program | — | (23) |
| Balance as of December 31, 2016 | 673 | (222) |
| Transactions in connection withstock-based compensation plans | — | 1 |
| Stock purchases under purchase programs | — | (19) |
| Balance as of December 31, 2017 | 673 | (240) |
| Stock purchases under purchase programs | — | (16) |
| Balance as of December 31, 2018 | 673 | (256) |

preferred stock we have 20 million shares of preferred stock authorized with a par value of $ 0.01 per share . no shares of preferred stock were outstanding as of december 31 , 2018 or 2017 . treasury stock we purchase shares of our common stock as authorized under our common stock purchase program ( described below ) and to meet our obligations under employee stock-based compensation plans . on july 13 , 2015 , our board of directors authorized us to purchase $ 2.5 billion of our outstanding common stock with no expiration date , and we completed that program during 2017 . on september 21 , 2016 , our board of directors authorized our purchase of up to an additional $ 2.5 billion with no expiration date , and we completed that program during 2018 . on january 23 , 2018 , our board of directors authorized our purchase of up to an additional $ 2.5 billion ( the 2018 program ) with no expiration date . during the years ended december 31 , 2018 , 2017 , and 2016 , we purchased $ 1.5 billion , $ 1.3 billion , and $ 1.3 billion , respectively , of our common stock under our programs . as of december 31 , 2018 , we have approval under the 2018 program to purchase approximately $ 2.2 billion of our common stock . common stock dividends on january 24 , 2019 , our board of directors declared a quarterly cash dividend of $ 0.90 per common share payable on march 5 , 2019 to holders of record at the close of business on february 13 , 2019 . valero energy partners lp units on september 16 , 2016 , vlp entered into an equity distribution agreement pursuant to which vlp offered and sold from time to time their common units having an aggregate offering price of up to $ 350 million based on amounts , at prices , and on terms determined by market conditions and other factors at the time of .

total_amount_spent = 2.5
number_of_months = 24
answer = total_amount_spent / number_of_months