Stacking multiplications 3

We have learned how to use stacking to do multiplications like 23 times 3 And 115 times 7. That is, multiplications where one factor consists of several digits, and the other factor of one single digit. What do we do when both factors have multiple digits? Let’s do the stacking first. We make sure the ones are stacked on top of each other, and the tens on top of each other.

Then we calculate, place value by place value. 2 times 4 is 8. 2 times 1 is 2. Now comes the new part. We multiply the tens digit with the number on top, one digit at the time.

First, 1 times 4. This is 4. We write four here. Then, 1 times 1. This is 1.

We write 1 here. And now there is one more thing to do. We add, column by column. Find the sum. 8 and … empty?

Yes, the box is empty, so we can write a zero there. 8 plus 0 is 8. Then 2 plus 4. This is 6. And then, finally, nothing plus 1.

This is 1. And now we are done. The product is 168. 14 times 12 is 168. There is a lot to keep track of, but as usual it will become easier and easier the more you practise.

So let’s look at one more calculation. 34 times 21. Let’s do the stacking first, like this. And now, we calculate. 1 times 4 is 4.

1 times 3 is 3. So it says 34 below the bar. Then we do 2 times 4, which is 8. And 2 times 3, which is 6. And we write a zero in the empty box.

And then we have the final step: add 34 and 680. This is ... 714. This is the product. 34 times 21 is 714.