In the previous post, we have learned the sum prediction math trick, a cool math trick that let’s you predict the sum of five numbers. In this post, we are going to learn why this math trick works. Needless to say, you need to read the previous post in order to understand the explanation below.

The steps in the previous post of this trick requires a friend to write a 3-digit number. The number 2 is then subtracted from this number. The number 2 is then appended at the first digit of the number. For example, if the number written is 893, we subtract 2, to get 891 and then append 2 to get 2891.

In the second step, another number is written by a friend and the one who predicts the sum must make sure that that number and the number he has written add up to 999. This is repeated in the fourth and fifth step.

**Why The Sum Prediction Math Trick Works**

In the first step, we subtracted 2 from the number and added 2 as the first digit. If this is a 3 digit number, adding 2 as the thousands digit is actually adding 2000. So, if we add 2000 and subtract 2, we are actually adding 1998.

So, to be able to get this result, we have to make sure that all the four remaining numbers add up to 1998.

But recall that in Step 3 and Step 5, we made sure that the two pairs of number add up to 999. And, 999 + 999 = 1998.

That’s why the math trick works. Amazing isn’t it?

Amaze Your Friends with The Sum Prediction Math Trick

If you want to impress your friends by reading their mind in advance, I bet that you want to learn this trick. This allows you to predict the sum in advance of the 5 numbers that you and your friend will write on a piece of paper. Note that the time your friend wrote the first number, you will already know the sum.

*Below are the steps of the sum prediction math trick.*

**Step 1**

Ask your friend to write a 3-digit number. For example, he wrote, 428.

During this step, even if you haven’t written the additional four numbers yet, you will already know the answer. The answer is 2426.

How to get 2426? Subtract 2 from the first 428 (becomes 426) and then place 2 as the first digit. Note that the first of the sum or total will always be 2!

**Step 2**

Ask him again to write another 3-digit number below what he had written. Tell him to align the digits. For example, he wrote 135.

**Step 3**

Write your 3-digit number such that the sum of the number in Step 2 and your number is 999. That means that you write the your number such that each digit add up to 9 with the number in Step 2. For example, 2 pairs with 7, 3 pairs with 6, and 5 pairs with 4.

So, the number that you will write is 764.

**Step 4**

Ask your friend again to write another 3-digit below the three numbers above. For example, he wrote 407.

**Step 5**

Again, write your number such that the one in Step 4 and your number add up to 999.

Now, impress your friend by adding all the numbers. The sum is 2426, the number you predicted in Step 1!

**Amazing isn’t it?**

The sum prediction math trick works for larger numbers. Just be sure that all the numbers have the same number of digits.