1. Peter doesn’t use the same combination on two days of this week.
2. Peter doesn’t use the same color on two consecutive days of this week.
3. Peter uses some color on four days.
4. Peter uses blue and black on Monday.
5. Peter uses green on Thursday.
6. Peter doesn’t use yellow on Friday.
7. Peter doesn’t combine red with black.
From rules 2 and 3, Peter must use one color on Monday, Wednesday, Friday and Sunday, which can be blue or black.
If it’s black, from rules 1 and 4, Peter must combine black with red, yellow, and green on Wednesday, Friday and Sunday, but rule 7 disallows combining black with red.
Therefore, it’s blue, and Peter must combine blue with red, yellow, and green on Wednesday, Friday, and Sunday. From rule 5, Peter uses green on Thursday, he can’t use green on Wednesday or Friday, and only on Sunday.
From rule 6, Peter doesn’t use yellow on Friday, and he can only use red on Friday and yellow on Wednesday.
Since Peter uses blue, red and green on Friday and Sunday, from rule 2, he can only use black and yellow on Saturday.
