Solve It In 5.18 Seconds

mental math series, part 5

5.18 seconds — that’s all the time it took to do the following calculation mentally.

And I’m confident you can learn this trick for two-by-two multiplication too.

So let’s get started!

Criss Cross Multiplication

Step 1: Multiply the one’s digits together. Write down the answer below the ones place.

Step 2: Multiply the diagonals and add them together.

In this problem, we have 2 x 2 = 4 and 1 x 3 = 3 as the diagonals. Add the results together to obtain 7 and write this value down next to 6.

Step 3: Multiply the digits in the ten’s column together and write the number to the left of the previous value.

That’s all there is to it! 23 x 12 = 276.

Example 2

You may have noticed that the products in the example above were single-digit answers. Our method only varies slightly if you come across a double-digit product along the way. Here’s an example to demonstrate.

Begin as before, multiplying the one’s digits together.

7 x 2 results in 14, which is a two-digit answer. Record the one’s digit of the product and carry the ten’s. I’ll mark the carry in blue.

Next multiply the diagonals together.

This time we have three number to add together: the product from the two diagonals as well as the 1 from the last step.

Add up the numbers marked in blue: 7 + 2 + 1 = 10. Replace the blue 1 with 10.

Lastly, multiply the digits in the ten’s column together.

We have 1 x 1 = 1. Combine 1 with the 1 from the previous step. The final digit to be placed next to the ten’s column is 2.

Hence the answer is 204.

Example 3

Don’t worry, you’ll get the hang of it with a little practice. Let’s try a more difficult example.

Begin by multiplying the one’s digits together: 8 x 5 = 40. Carry the 4.

Multiply the diagonals.

Add all the blue numbers together and write the answer to the left of the zero (35 + 16 + 4 = 35 + 20 = 55).

Multiply the ten’s digits together.

Add the blue numbers together (14 + 5) and replace the left most 5 with the sum.

This method can be extended to any 3-by-3, 4-by-4, … n-by-n multiplication problem with a little work.

Next Lesson: Find Perfect Squares Mentally With This Trick

Thanks for reading!

Please click the ❤ to let me know you learned something new!