Mira compares the January and April pages of the same year. For how many dates d from 1 to 30 do d January and d April fall on the same day of the week?

From 1 January to 1 April of an ordinary year is 31 + 28 + 31 = 90 days = 12 weeks + 6 days, so every April date is 6 days on from the same January date. No date matches: 0. (30 is the answer only in a leap year, when the gap is 91 days.)