{"pre_text": ["our refining and wholesale marketing gross margin is the difference between the prices of refined products sold and the costs of crude oil and other charge and blendstocks refined , including the costs to transport these inputs to our refineries , the costs of purchased products and manufacturing expenses , including depreciation .", "the crack spread is a measure of the difference between market prices for refined products and crude oil , commonly used by the industry as a proxy for the refining margin .", "crack spreads can fluctuate significantly , particularly when prices of refined products do not move in the same relationship as the cost of crude oil .", "as a performance benchmark and a comparison with other industry participants , we calculate midwest ( chicago ) and u.s .", "gulf coast crack spreads that we feel most closely track our operations and slate of products .", "posted light louisiana sweet ( 201clls 201d ) prices and a 6-3-2-1 ratio of products ( 6 barrels of crude oil producing 3 barrels of gasoline , 2 barrels of distillate and 1 barrel of residual fuel ) are used for the crack spread calculation .", "our refineries can process significant amounts of sour crude oil which typically can be purchased at a discount to sweet crude oil .", "the amount of this discount , the sweet/sour differential , can vary significantly causing our refining and wholesale marketing gross margin to differ from the crack spreads which are based upon sweet crude .", "in general , a larger sweet/sour differential will enhance our refining and wholesale marketing gross margin .", "in 2009 , the sweet/sour differential narrowed , due to a variety of worldwide economic and petroleum industry related factors , primarily related to lower hydrocarbon demand .", "sour crude accounted for 50 percent , 52 percent and 54 percent of our crude oil processed in 2009 , 2008 and 2007 .", "the following table lists calculated average crack spreads for the midwest ( chicago ) and gulf coast markets and the sweet/sour differential for the past three years .", "( dollars per barrel ) 2009 2008 2007 ."], "post_text": ["sweet/sour differential ( a ) $ 5.82 $ 11.99 $ 11.59 ( a ) calculated using the following mix of crude types as compared to lls. : 15% ( 15 % ) arab light , 20% ( 20 % ) kuwait , 10% ( 10 % ) maya , 15% ( 15 % ) western canadian select , 40% ( 40 % ) mars .", "in addition to the market changes indicated by the crack spreads and sweet/sour differential , our refining and wholesale marketing gross margin is impacted by factors such as : 2022 the types of crude oil and other charge and blendstocks processed , 2022 the selling prices realized for refined products , 2022 the impact of commodity derivative instruments used to manage price risk , 2022 the cost of products purchased for resale , and 2022 changes in manufacturing costs , which include depreciation .", "manufacturing costs are primarily driven by the cost of energy used by our refineries and the level of maintenance costs .", "planned turnaround and major maintenance activities were completed at our catlettsburg , garyville , and robinson refineries in 2009 .", "we performed turnaround and major maintenance activities at our robinson , catlettsburg , garyville and canton refineries in 2008 and at our catlettsburg , robinson and st .", "paul park refineries in 2007 .", "our retail marketing gross margin for gasoline and distillates , which is the difference between the ultimate price paid by consumers and the cost of refined products , including secondary transportation and consumer excise taxes , also impacts rm&t segment profitability .", "there are numerous factors including local competition , seasonal demand fluctuations , the available wholesale supply , the level of economic activity in our marketing areas and weather conditions that impact gasoline and distillate demand throughout the year .", "refined product demand increased for several years until 2008 when it decreased due to the combination of significant increases in retail petroleum prices , a broad slowdown in general economic activity , and the impact of increased ethanol blending into gasoline .", "in 2009 refined product demand continued to decline .", "for our marketing area , we estimate a gasoline demand decline of about one percent and a distillate demand decline of about 12 percent from 2008 levels .", "market demand declines for gasoline and distillates generally reduce the product margin we can realize .", "we also estimate gasoline and distillate demand in our marketing area decreased about three percent in 2008 compared to 2007 levels .", "the gross margin on merchandise sold at retail outlets has been historically less volatile. ."], "table": [["( dollars per barrel )", "2009", "2008", "2007"], ["chicago lls 6-3-2-1", "$ 3.52", "$ 3.27", "$ 8.87"], ["u.s . gulf coast lls 6-3-2-1", "$ 2.54", "$ 2.45", "$ 6.42"], ["sweet/sour differential ( a )", "$ 5.82", "$ 11.99", "$ 11.59"]], "table_ori": [["<i>(Dollars per barrel)</i>", "2009", "2008", "2007"], ["Chicago LLS 6-3-2-1", "$3.52", "$3.27", "$8.87"], ["U.S. Gulf Coast LLS 6-3-2-1", "$2.54", "$2.45", "$6.42"], ["Sweet/Sour differential<sup>(a)</sup>", "$5.82", "$11.99", "$11.59"]]}

Question: by what percentage did the average crack spread for the midwest ( chicago ) decrease from 2007 to 2009?
Evidence:
table_1: ( dollars per barrel ) the chicago lls 6-3-2-1 of 2009 is $ 3.52 ; the chicago lls 6-3-2-1 of 2008 is $ 3.27 ; the chicago lls 6-3-2-1 of 2007 is $ 8.87 ;
Program: subtract(3.52, 8.87), divide(#0, 8.87)
Answer:
str_answer: -60.3%
exe_answer: -0.60316