One hundred days after a Monday

Question

Today is a Monday. What day of the week will it be exactly 100 days from today? Do it in your head, without a calendar, and explain the method so that it works for any number of days, including very large ones such as a million days from now.

Accepted Answer

Wednesday. Method. Every 7 days the weekday comes back to Monday, so all that matters is the remainder when 100 is divided by 7. Since 7 × 14 = 98, the remainder is 2. Two days after Monday is Wednesday. For any number of days. Divide by 7, keep the remainder r, and count r days forward from today. For 1,000 days: 7 × 142 = 994, remainder 6, so six days after Monday is Sunday. For a year of 365 days: remainder 1, which is why your birthday moves forward one weekday each ordinary year and two in a leap year. A trick for large numbers. Remainders multiply, so you can reduce in pieces. Ten leave…

One hundred days after a Monday

Your answer

Discussion

Learn the concepts

Related questions