Answer
(a) Sunday, correct.
(b) Sunday, correct.
(c) Tuesday, correct.
Work Step by Step
Formula (4.4.1) states that DayN=(DayT+N)mod 7
(a) For DayT=6, N=15, we have DayN=(6+15)mod 7=(21)mod 7=0 or Sunday which is correct.
(b) For DayT=0, N=7, we have DayN=(0+7)mod 7=(7)mod 7=0 or Sunday which is correct.
(c) For DayT=4, N=12, we have DayN=(4+12)mod 7=(16)mod 7=2 or Tuesday which is correct.