Traditionally, multiplication of multiple digit numbers is done as a series of multiplications that are eventually added together to form a final answer. The criss-cross method is a variation on this technique that allows for much quicker processing of the problem without the need for a calculator or extensive use of paper space. There are many situations, such as trips to the grocery store, where you will find a need to perform multiplication of odd numbers in order to stay within a budget as you shop.
This system of multiplication is adopted from Vedic Mathematics' URDHVA-TIRYAK SUTRA, which means vertically and cross-wise.
To start with, we will look at a simple example just to get a grasp on the steps involved in the method. Later we will apply it to a slightly more advanced problem to show how to handle carrying numbers from one digit to the next. For now, we will multiply 111 by 111.
First, you will take the right-hand digits and multiply them together. This will give you the one's digit of the answer, as shown below with the digits used encased in brackets.
* - -- 1
Next, multiply the one's digit of the top number by the ten's digit of the bottom number, and the one's digit of the bottom number by the ten's digit of the top number. Once you have those values, add them together, and you will have the ten's digit of the answer. The digits you multiply together are enclosed in the same type of bracket. This gives (1*1)+(1*1), so the ten's digit is equal to two.
For the next step, all digits of the number will be involved in order to find the middle of the answer. Multiply the one's digit of one to the hundreds digit of the other, and then multiply the ten's digit of both together, then finally add them all together. This will give you (1*1)+(1*1)+(1*1) for a value of 3. As above, the digits paired together are enclosed in the same type of bracket.
The fourth step is similar to the second step, just moved one place to the left. You multiply the ten's digit of one number by the hundred's digit of the other number. Again you will get (1*1) + (1*1), showing the thousand's digit of the answer is equal to 2.
For the final step, simply multiply the left-hand number of both numbers together to get a value of 1 for the ten-thousand's place.
If you are a teacher you can go ahead and teach this wonderful teaching technique to your folks. If not, teaching certification programs like the St Joseph University online program offer aspiring math teachers a way to earn a degree at their own pace.
To give a real-world example, consider that you want to buy 15 of some product for $1.25 each. You can consider 15 to be a three digit number, where the third digit is equal to 0.
* - -- -
You will notice that immediately you will have to deal with carrying over value from one digit to the other. This works very similar to regular multiplication methods. You simply take any value in the tens digit of that step as an addition to the next step. When you multiply 5 by 5 in this example to get 25, you would place the 5 as the one's digit of the answer, and add the 2 to the next step to find the ten's digit. This makes the second step equal to (5*2) + (1*5) +2, for a total of 17. Use the 7 as the value for the ten's digit, and carry the one over to the next step. Here you would end up with (5*1) + (1*2) + (0*5) + 1, coming to a value of 8. This time there is no number to carry over, so proceed through the rest of the problem as normal. The thousands place is 1 via (0*2)+(1*1), and the ten-thousands place is equal to 0 via 0*1. This comes to 01875, then drop the 0 from the end.
As in regular multiplication, you count the total number of places behind the decimal point, and add the same number to the answer. This means the items would cost $18.75 to purchase.
Justin McGenity is a freelance writer, science geek, and self-proclaimed philosopher. His hobbies include studying, video games, and socializing. You can find him writing for such sites as Degree Jungle a resource for university students.