On Monday, Mr Castiel ate 2/5*600 = <<2/5*600=240>>240 sausages.
The remaining number of sausages after he ate 240 on Monday is 600-240 = <<600-240=360>>360
On Tuesday, he ate half of the remaining sausages, a total of 360/2 = <<360/2=180>>180.
The remaining number of sausages that he saved for Friday after eating 180 on Tuesday is 360-180 = <<360-180=180>>180.
On Friday, Mr Castiel ate 3/4*180 = <<3/4*180=135>>135 of the sausages he had kept.
The remaining number of sausages after he ate 135 on Friday is 180-135 = <<180-135=45>>45
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