question: The cell-phone recharges at the rate of 1 percentage-point of charge per 3 minutes. Now, the phone is at 60% charged. How long will it take to fully charge, in hours?
answer: 100-60=<<100-60=40>>40 percentage points.
  Then 40*3=<<40*3=120>>120 minutes to fully charge.
  And 120/60=<<120/60=2>>2 hours.
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