An analyst examines the effect that various variables have on crop yield.He estimates the following model: 
,where y is the average yield in bushels per acre,x1 is the amount of summer rainfall,x2 is the average daily use in machine hours of tractors on the farm,and x3 is the amount of fertilizer used per acre.A portion of the regression results is shown in the accompanying table. 
a.Predict the crop yield per acre if x1 is 5,x2 is 4,and x3 is the 0.5.
b.Calculate the standard deviation of the difference between the actual crop yield and the estimate of the crop yield.
c.How much of the variation in crop yield is unexplained by the model?
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