What day will it be?

Hello, and welcome to Myriora Maths! Today is Monday. What day will it be in one hundred days? You could count on your fingers for a long time. Or, you could use a shortcut.

Days repeat every seven. Seven days after Monday is Monday again. So is fourteen days, twenty-one days, or any multiple of seven.

So, divide by seven, and look at the remainder. One hundred divided by seven is fourteen, remainder two. Fourteen whole weeks bring us back to Monday. Then two more days: Wednesday!

Going backwards works the same way. What day was it fifty days ago? Fifty divided by seven is seven, remainder one. One day back from Monday is Sunday.

Months are trickier, because they're different lengths. Thirty days have September, April, June and November. All the rest have thirty-one, except February, with twenty-eight, or twenty-nine in a leap year.

If the first of March is a Tuesday, what day is the first of April? March has thirty-one days. Thirty-one divided by seven leaves three. Tuesday, plus three days: Friday!

A common mistake is counting today as day one. In one hundred days means moving one hundred days on, so start counting from tomorrow.

Your turn! Today is Friday. What day will it be in sixty days? You've got twenty seconds, or pause the video if you need more time. Go!

Time's up! Sixty divided by seven is eight, remainder four. Friday, plus four days: Tuesday!

Quick review. Days repeat every seven. Divide by seven, and only the remainder matters. Then count forward, or back, by the remainder. See you next time!