On Tuesday, Tristan studies twice as long as he did on Monday and this amounts to 4 hours * 2 = <<4*2=8>>8 hours.
So in the first 5 days of the week, he studies for a total of 4 hours Monday + 8 hours Tuesday + 3 hours Wednesday + 3 hours Thursday + 3 hours Friday = 21 hours.
He therefore has 25 planned hours – 21 used hours = <<25-21=4>>4 hours left to study.
As he splits his time evenly over 2 days, he spends 4 hours / 2 = <<4/2=2>>2 hours studying on Saturday.
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