item 7a . quantitative and qualitative disclosures about market risk ( amounts in millions ) in the normal course of business , we are exposed to market risks related to interest rates , foreign currency rates and certain balance sheet items . from time to time , we use derivative instruments , pursuant to established guidelines and policies , to manage some portion of these risks . derivative instruments utilized in our hedging activities are viewed as risk management tools and are not used for trading or speculative purposes . interest rates our exposure to market risk for changes in interest rates relates primarily to the fair market value and cash flows of our debt obligations . the majority of our debt ( approximately 86% ( 86 % ) and 94% ( 94 % ) as of december 31 , 2018 and 2017 , respectively ) bears interest at fixed rates . we do have debt with variable interest rates , but a 10% ( 10 % ) increase or decrease in interest rates would not be material to our interest expense or cash flows . the fair market value of our debt is sensitive to changes in interest rates , and the impact of a 10% ( 10 % ) change in interest rates is summarized below . increase/ ( decrease ) in fair market value as of december 31 , 10% ( 10 % ) increase in interest rates 10% ( 10 % ) decrease in interest rates .

increase/ ( decrease ) in fair market value 10% ( 10 % ) increasein interest rates: 2018: -91.3
2017: -20.2
increase/ ( decrease ) in fair market value 10% ( 10 % ) decreasein interest rates: 2018: 82.5
2017: 20.6

we have used interest rate swaps for risk management purposes to manage our exposure to changes in interest rates . we did not have any interest rate swaps outstanding as of december 31 , 2018 . we had $ 673.5 of cash , cash equivalents and marketable securities as of december 31 , 2018 that we generally invest in conservative , short-term bank deposits or securities . the interest income generated from these investments is subject to both domestic and foreign interest rate movements . during 2018 and 2017 , we had interest income of $ 21.8 and $ 19.4 , respectively . based on our 2018 results , a 100 basis-point increase or decrease in interest rates would affect our interest income by approximately $ 6.7 , assuming that all cash , cash equivalents and marketable securities are impacted in the same manner and balances remain constant from year-end 2018 levels . foreign currency rates we are subject to translation and transaction risks related to changes in foreign currency exchange rates . since we report revenues and expenses in u.s . dollars , changes in exchange rates may either positively or negatively affect our consolidated revenues and expenses ( as expressed in u.s . dollars ) from foreign operations . the foreign currencies that most favorably impacted our results during the year ended december 31 , 2018 were the euro and british pound sterling . the foreign currencies that most adversely impacted our results during the year ended december 31 , of 2018 were the argentine peso and brazilian real . based on 2018 exchange rates and operating results , if the u.s . dollar were to strengthen or weaken by 10% ( 10 % ) , we currently estimate operating income would decrease or increase approximately 4% ( 4 % ) , assuming that all currencies are impacted in the same manner and our international revenue and expenses remain constant at 2018 levels . the functional currency of our foreign operations is generally their respective local currency . assets and liabilities are translated at the exchange rates in effect at the balance sheet date , and revenues and expenses are translated at the average exchange rates during the period presented . the resulting translation adjustments are recorded as a component of accumulated other comprehensive loss , net of tax , in the stockholders 2019 equity section of our consolidated balance sheets . our foreign subsidiaries generally collect revenues and pay expenses in their functional currency , mitigating transaction risk . however , certain subsidiaries may enter into transactions in currencies other than their functional currency . assets and liabilities denominated in currencies other than the functional currency are susceptible to movements in foreign currency until final settlement . currency transaction gains or losses primarily arising from transactions in currencies other than the functional currency are included in office and general expenses . we regularly review our foreign exchange exposures that may have a material impact on our business and from time to time use foreign currency forward exchange contracts or other .

Questions:
in the year of 2018, what was the impact to the fair market value of the 10% ( 10 % ) increase in interest rates?
and what was that for the decrease in interest rates?
how much, then, did the increase amount represent in relation to this decrease one?
and for that year and the one before, what was the total interest income, in millions?
and what was the average between the two years?

Answers:
-91.3
82.5
-1.1
41.2
20.5