How to Multiply 2 Digit Numbers by 9 Really Fast

We have had several posts regarding lightning speed multiplication and in this post, we are going to learn another trick on how to multiply by 9. Note that this method is only possible if the ones digit is greater than the tens digit.

How to Multiply 2 Digit Numbers by 9 Fast

To get the product of 2 digit number multiplied by 9

1.) Copy the tens digit of the 2-digit number as the hundreds digit of the product.

2.) Count the numbers between the the tens and ones digit of the 2-digit number and make it the tens digit of the product.

3.) Count how many numbers from the ones digit of the 2-digit number up to 10 including 10, and then make it the ones digit of the product.  Continue reading

How to Estimate Square Roots

In this post, we are going to learn a new trick. This is on how to estimate square roots. Suppose that you forgot your calculator and you have to calculate square root. Although you have no calculator, you are familiar with may perfect squares. For example, 13^2 (thirdteen squared) is 169 and 8^2 = 64. Let’s see how we can use this stock knowledge on estimating square roots.

How to Estimate Square Roots

Example 1: How do we estimate the square root of 87?

Step 1: Get the nearest perfect square and get its square root. In this case, we have 81 which has a square root of 9.

Step 2: Divide the number by square root obtained in Step 1. So, we have 87/9 = 9.6667

Step 3: Average the result in Step 1 and Step 2. That is

(9 + 9.6667)/2 =9.3333

If you use your calculator, the square root of 87 is equal to 9.3273.  Continue reading

2015 Year in Review – Second Quarter

Starting today aside from the monthly reviews, we will also be having reviews at the end of every quarter. This is to go back to all the posts for the last three months.

First, learn the 1089 Magic trick and learn how it works.

Secondly, learn how to square 2 digit numbers mentally from 1 to 100.

Next, amaze your friends with the sum prediction trick and teach the close ones how it works.  Continue reading

Month in Review – June 2015

Starting last month, at the end of the month, or the first week of the month, I will be reviewing the posts for the whole month. In this post, we review what we have learned so far last month, June 2015.

June 2015 in Review – Summary of Posts

Continue reading