She wants to read 200+400+300=<<200+400+300=900>>900 pages total.
She needs to spend 900/100 = <<900/100=9>>9 hours reading in the next three weeks.
In minutes, she will need to spend 9*60 = <<9*60=540>>540 minutes.
Thus, she should spend 540 minutes/10 days = <<540/10=54>>54 minutes per day reading.
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