Arthur D. answered 04/11/14
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Here's a simple solution.
2100 miles in 7 days
2100/7=300 miles per day on average
there are 7 days so the 4th day is the middle day when the balloon traveled 300 miles
each day the balloon traveled 50 more miles
300, 350, 400, 450
300, 250, 200, 150
150, 200, 250, 300, 350, 400, 450 for days 1,2,3,4,5,6, and 7