Answer:
10 minutes.
Explanation:
Nancy walked a total of 50 minutes * 4 days = <<504=200>>200 minutes last week.
Gillian walked a total of 35 minutes * 6 days = <<356=210>>210 minutes last week.
The total number of minutes that Gillian walked is greater than the total number of minutes that Nancy walked by 210 minutes - 200 minutes = <<210-200=10>>10 minutes.