Mental Multiplication: 2 Digit Numbers by 1 Digit Numbers

Mental multiplication is hard, but there are strategies and tricks that you can use to do this.  In a previous post, we have learned an alternative way on how to multiply 2-digit number by 2-digit number faster, and in this post, I am going to share another way, on how to do mental multiplication of of 2-digit numbers by 1-digit numbers. In this strategy, we will split the numbers into tens and ones, multiply separately, then add the products. Let’s have our first example.

The Math Trick

Example 1: 25 × 7

In this example, we split 25 into tens and ones 20 and 5. Then, we multiply both of them by 7.

Step 1: 20 × 7 = 140

Step 2: 5 × 7 = 35

Adding both products, we have  Continue reading

How to Multiply by 9, 99, 999, 9999 Faster

Before I explain the previous post about multiplication, I would like to discuss first multiplication by 9, 99, 999, 9999 or any number whose digits are all 9. For example, how do we multiply 87 × 99? To multiply by numbers whose digits are all 9, we just add 1 to 9, 99, 999, 9999 and so on and then subtract the number that we are multiplying with it.

Example 1: 7 × 9

1. 7 × 9 = 7 × 10 – 7

= 70 – 7 = 63

The reason that we subtract 7 is that because 9 = 10 – 1 and that 7 × 9 = 7 (10 – 1) = 70 – 7 = 63.

Example 2: 4 × 99

4 × 99 = 4(100 – 1) = 400 – 4 = 396

Example 3: 18 × 999

18 × 999 = 18(1000 – 1) = 18000 – 18 = 17982  Continue reading

Multiplying 2 Digit Numbers Fast

The usual steps that we in multiplying 2 digit numbers is to multiply vertically as shown in the first figure. We first use the digits ones digit of the multiplicand (12) and distribute to the ones and tens digit of the multiplier (34) shown in Step 1 and Step 2 and write the products. Then, we multiply the tens digit of the multiplicand to the ones and tens digit of the multiplier (Steps 3 and 4). Finally, we add the products to get the final answer.

multplying 2 digit numbers

As you can see, this method requires 5 steps. The discussion below is about an alternative method. Continue reading

Mind Reading Trick 2: The Answer is 4

This is the second mind reading trick in this blog. The first one was titled “The Answer is 3!

The Mind Reading Trick

Step 1: Think of a number.

For example, we think of 8.

Step 2: Multiply it by 3.

Now, 8 multiplied by 3 equals 24.

Step 3: Add 12.

12 + 24 = 36

Step 4: Divide by 3.

36/3 = 12

Step 5: Subtract the original number. The answer is 4.

12 – 8 = 4.

Note that whatever number you chose in Step 1, the final answer after step 5 will always be 4. If you don’t believe me try other numbers. :) Continue reading