question: Mark plants a beanstalk below his second-story window, which is 20 feet off the ground. The beanstalk doubles its height every day. If the beanstalk starts out 4 feet tall, how many days will it take to be taller than Mark's window?
answer: To find the beanstalk's height after one day, multiply the starting height by 2: 4 feet * 2 = <<4*2=8>>8 feet
  To find its height after two days, multiply the height after one day by 2: 8 feet * 2 = <<8*2=16>>16 feet
  To find its height after three days, multiply the height after two days by 2: 16 feet * 2 = <<16*2=32>>32 feet. This is taller than the window, so it took three days for the beanstalk to pass the window.
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