question: Madeline and her roommate, Keenan, share the cost of groceries. In total, they spend about $400 per month. In a four-week month, how many dollars does Keenan spend per week if Madeline pays 60% of the cost?
answer: They spend $400/4=$<<400/4=100>>100 per week.
  Keenan pays for 100-60=<<100-60=40>>40% of the groceries.
  Thus, she spends $100*(40/100)=$<<100*40/100=40>>40 per week.
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