× 5, × 25, × 99

Hello, and welcome to Myriora Maths! Forty-eight times five. Thirty-six times twenty-five. Twenty-three times ninety-nine. They look hard, but each one has a shortcut!

Times five is the same as times ten, then halve. Forty-eight times ten is four hundred and eighty. Half of that is two hundred and forty. So forty-eight times five is two hundred and forty!

Times twenty-five is times one hundred, then divide by four. Thirty-six times one hundred is three thousand, six hundred. To divide by four, halve it, and halve it again. One thousand, eight hundred, then nine hundred. So thirty-six times twenty-five is nine hundred!

Times ninety-nine is times one hundred, then take away one lot of the number. Twenty-three times one hundred is two thousand, three hundred. Take away one twenty-three. Two thousand, two hundred and seventy-seven!

Why do these work? Five is half of ten. Twenty-five is a quarter of one hundred, just like four lots of twenty-five pence make a pound. And ninety-nine is one less than one hundred. So we multiply by the easy number, then fix it.

A common mistake with times ninety-nine is taking away one, instead of one lot of the number. Twenty-three times one hundred, take away one, is two thousand, two hundred and ninety-nine. That's wrong! We need to take away a whole twenty-three, so it's two thousand, two hundred and seventy-seven.

Your turn! What is sixty-four times twenty-five? You've got twenty seconds, or pause the video if you need more time. Go!

Time's up! Sixty-four times one hundred is six thousand, four hundred. Divide by four: one thousand, six hundred!

Quick review. Times five: times ten, then halve. Times twenty-five: times one hundred, then divide by four. Times ninety-nine: times one hundred, then take away one lot. See you next time!