Sam makes $460 / 23 hrs = $<<460/23=20>>20/hr.
From September to February, Sam made 8hrs x $20/hr = $<<8*20=160>>160.
From March to February, Sam made a total of $460 + $160 = $620.
After fixing his car, he was left with $620 - $340 = $<<620-340=280>>280.
Sam needs another $600 - $280 = $<<600-280=320>>320.
Sam needs to work another $320 / $20/hr = <<320/20=16>>16 hours.
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