| Question | What was the day of the week on June 17, 1991? | |
| Type | multiple_choice | |
| Option | Tuesday | incorrect |
| Option | Wednesday | incorrect |
| Option | Friday | incorrect |
| Option | Monday | correct |
| Solution | June 17, 1991 = (1990 years + period from 01.01.1991 to 17.06.91) ⇒Odd days in 1600 years = 0 ⇒Odd days in the next 300 years = 15 odd days (2 week + 1 odd day) = 1 odd day 90 years have 22 leap years + 68 ordinary years. ⇒Number of odd days in 90 years = 22*2 + 68*1 = 112 odd days (16 weeks + 0 odd day) = 0 odd day Number of odd days from 01.01.91 to 17.06.91= Jan. (31) + Feb. (28) + March (31) + April (30) + May (31) + June (17) = 168 days ⇒168 days = 24 weeks + 0 odd days Total number of odd days = 0 + 1 + 0+ 0 = 1 odd day 1 odd day represents Monday, so the given day was Monday. | |
