Adding lots of numbers fast

Hello, and welcome to Myriora Maths! What is seventeen, plus twenty-six, plus thirteen, plus four, plus twenty-four? Five numbers! Let's see how to add them fast.

First, look at the ones digits. Seventeen ends in seven, and thirteen ends in three. Seven and three make ten, so seventeen plus thirteen is a round number: thirty. Twenty-six ends in six, and twenty-four ends in four: together, they make fifty.

Now it's easy. We have thirty, fifty, and the four on its own. Thirty plus fifty is eighty. And eighty plus four is eighty-four!

Why does this work? Adding in any order gives the same total. So we choose the order that makes round numbers. And round numbers are easy to add in your head.

Here's another trick: adding nine. Nine is one less than ten. So to add nine, add ten, then take away one. Forty-six plus nine: forty-six plus ten is fifty-six, take away one is fifty-five.

The same works for nineteen, twenty-nine, and so on. Thirty-four plus nineteen: add twenty, that's fifty-four, then take away one: fifty-three.

Let's try one together. What is eight, plus fifteen, plus two, plus five? Here's a tip: look for pairs that make round numbers. Say the answer out loud!

Eight plus two is ten. Fifteen plus five is twenty. Ten plus twenty is thirty! Well done if you said thirty!

Here's a question that catches lots of people. What is thirty-eight plus nine? Some people add ten, and forget to take one away: forty-eight. But we added one too many! Take it away: forty-seven.

Your turn! What is twenty-three, plus nineteen, plus seventeen? Pick the right card. You've got twenty seconds, or pause the video. Go!

Time's up! Twenty-three plus seventeen is forty. Then add nineteen: add twenty, that's sixty, then take away one. It's fifty-nine!

Quick review. Look at the ones digits, and pair numbers that make round numbers. First, add the round numbers. And to add nine, add ten and take away one. See you next time!