Doubling and halving

Hello, and welcome to Myriora Maths! What is sixteen times twenty-five? It looks hard. But with one simple trick, you can do it in your head!

Here's the trick: halve one number, and double the other. Half of sixteen is eight. Double twenty-five is fifty. So sixteen times twenty-five is the same as eight times fifty!

Why does the answer stay the same? Here are four rows of six dots. Cut the array in half, and put the two halves side by side. Now it's two rows of twelve. There are half as many rows, but each row is twice as long. It's the same dots, so it's the same answer: twenty-four.

Let's keep going! Halve and double again: four times one hundred. Now it's easy. Four times one hundred is four hundred! So sixteen times twenty-five is four hundred.

This trick works best when doubling makes a round number. Five doubles to ten. Twenty-five doubles to fifty, then a hundred. And fifty doubles to a hundred.

Let's try one together. What is fourteen times five? Halve the fourteen, and double the five. Say it out loud!

Half of fourteen is seven. Double five is ten. Seven times ten is seventy!

Here's a question that catches lots of people. Some people double both numbers: sixteen times twenty-five becomes thirty-two times fifty. But that makes the answer four times too big! Halve one number, and double the other: then the answer stays the same.

Your turn! What is eighteen times five? Pick the right card. You've got fifteen seconds, or pause the video. Go!

Time's up! Half of eighteen is nine, and double five is ten. Nine times ten is ninety. Eighteen times five is ninety!

Quick review. Halve one number, and double the other: the answer stays the same. Keep going until it's easy. It works best with fives, twenty-fives and fifties. See you next time!