100 Days Later
Read the puzzle
Seven day tiles, MON, TUE, WED, THU, FRI, SAT and SUN, sit in a ring around a centre card that reads: Today is Saturday. Partway through, the video points out that every 7 days you land on the same day again. What day will it be 100 days from today?
What day will it be 100 days from today?
Take away whole weeks: 100 = 98 + 2.
Monday
100 = 14 weeks (98 days) + 2 days. Saturday + 2 = Monday.
How to solve it
Step one: strip out whole weeks. Every 7 days the week comes back to the same day, so full weeks do not change the answer. 7 x 14 = 98, which is the largest multiple of 7 that fits into 100. That leaves 100 - 98 = 2 days.
Step two: 98 days from Saturday is Saturday again. Now count the 2 leftover days forward: Saturday + 1 is Sunday, Saturday + 2 is Monday.
So 100 days from today is a Monday.
The usual wrong answer is Sunday, which comes from counting today as day 1 and so moving only one day past Saturday. Today is day 0; tomorrow is the first day from today. Another slip is dividing 100 by 7, getting 14 remainder 2, and then counting the 2 from Sunday instead of from Saturday.
Takeaway: for any day-of-the-week jump, take the remainder after dividing by 7 and step that many days forward from today. For example, 30 days from a Saturday is 30 - 28 = 2 days on, so also a Monday.
Quick check: 14 weeks and 2 days is 14 x 7 + 2 = 100 days.
