{"pre_text": ["crude oil , and political unrest in the middle east and elsewhere .", "later in 2008 , crude oil prices dropped more rapidly than they had climbed as the u.s .", "dollar rebounded and other countries entered recessions which decreased demand .", "during 2008 , the average spot price per barrel for wti was $ 99.75 , up from an average of $ 72.41 in 2007 , but ended the year at $ 44.60 .", "the average spot price per barrel for brent was $ 97.26 in 2008 , up from an average of $ 72.39 in 2007 , but ended the year at $ 36.55 .", "the differential between wti and brent average prices widened to $ 2.49 in 2008 from $ 0.02 in 2007 .", "our domestic crude oil production is on average heavier and higher in sulfur content than light sweet wti .", "heavier and higher sulfur crude oil ( commonly referred to as heavy sour crude oil ) sells at a discount to light sweet crude oil .", "our international crude oil production is relatively sweet and is generally sold in relation to the brent crude oil benchmark .", "natural gas prices on average were higher in 2008 than in 2007 .", "a significant portion of our u.s .", "lower 48 states natural gas production is sold at bid-week prices or first-of-month indices relative to our specific producing areas .", "the average henry hub first-of-month price index was $ 2.18 per thousand cubic feet ( 201cmcf 201d ) higher in 2008 than the 2007 average .", "natural gas sales in alaska are subject to term contracts .", "our other major natural gas-producing regions are europe and equatorial guinea , where large portions of our natural gas sales are subject to term contracts , making realized prices in these areas less volatile .", "as we sell larger quantities of natural gas from these regions , to the extent that these fixed prices are lower than prevailing prices , our reported average natural gas prices realizations may decrease .", "e&p segment income during 2008 was up 57 percent from 2007 , with revenue increases tied to these increases in average commodity prices accounting for almost half of the income improvement .", "liquid hydrocarbon and natural gas sales volumes were also higher in 2008 than 2007 .", "oil sands mining oil sands mining segment revenues correlate with prevailing market prices for the various qualities of synthetic crude oil and vacuum gas oil we produce .", "roughly two-thirds of the normal output mix will track movements in wti and one-third will track movements in the canadian heavy sour crude oil marker , primarily western canadian select .", "output mix can be impacted by operational problems or planned unit outages at the mine or upgrader .", "during 2008 , our average realized price for synthetic crude oil and vacuum gas oil was $ 91.90 per barrel , up from 2007 , but ended the year at $ 24.97 per barrel impacted by a heavier yield in december and a seasonal decrease in the value of our heavy output .", "the operating cost structure of the oil sands mining operations is predominantly fixed , and therefore many of the costs incurred in times of full operation continue during production downtime .", "per unit costs are sensitive to production rates .", "key variable costs are natural gas and diesel fuel , which track commodity markets such as the canadian aeco natural gas sales index and crude prices respectively .", "the table below shows benchmark prices that impact both our revenues and variable costs , listing high and low spot prices during the year. ."], "post_text": ["wti crude oil ( dollars per barrel ) $ 145.29 july 3 $ 33.87 december 19 western canadian select ( dollars per barrel ) ( a ) $ 114.95 july $ 23.18 december aeco natural gas sales index ( canadian dollars per gigajoule ) ( b ) $ 11.34 july 1 $ 5.42 september 19 ( a ) monthly pricing based upon average wti adjusted for differentials unique to western canada .", "( b ) alberta energy company day ahead index .", "our osm segment reported income of $ 258 million for 2008 , reflecting synthetic crude oil and vacuum gas oil sales averaging 32 mboepd .", "derivative instruments intended to hedge price risk on future sales have impacted revenues in the periods presented , with net gains of $ 48 million in 2008 and net losses of $ 53 million in 2007 .", "in the first quarter of 2009 , we entered into derivative instruments which effectively offset certain of our open derivative positions .", "refining , marketing and transportation rm&t segment income depends largely on our refining and wholesale marketing gross margin , refinery throughputs , retail marketing gross margins for gasoline , distillates and merchandise , and the profitability of our pipeline transportation operations. ."], "table": [["benchmark wti crude oil ( dollars per barrel )", "high $ 145.29", "date july 3", "low $ 33.87", "date december 19"], ["western canadian select ( dollars per barrel ) ( a )", "$ 114.95", "july", "$ 23.18", "december"], ["aeco natural gas sales index ( canadian dollars per gigajoule ) ( b )", "$ 11.34", "july 1", "$ 5.42", "september 19"]], "table_ori": [["Benchmark", "High", "Date", "Low", "Date"], ["WTI crude oil<i>(Dollars per barrel)</i>", "$145.29", "July 3", "$33.87", "December 19"], ["Western Canadian Select<i>(Dollars per barrel)</i><sup>(a)</sup>", "$114.95", "July", "$23.18", "December"], ["AECO natural gas sales index<i>(Canadian dollars per gigajoule)</i><sup>(b)</sup>", "$11.34", "July 1", "$5.42", "September 19"]]}

Question: during 2008 , what was the change in average realized price for synthetic crude oil and vacuum gas oil between the end of 2007 and the end of 2008 , per barrel?
Evidence:
text_21: during 2008 , our average realized price for synthetic crude oil and vacuum gas oil was $ 91.90 per barrel , up from 2007 , but ended the year at $ 24.97 per barrel impacted by a heavier yield in december and a seasonal decrease in the value of our heavy output .
Program: subtract(91.90, 24.97)
Answer:
str_answer: 66.93
exe_answer: 66.93