Why The Sum Prediction Math Trick Works

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. 

sum prediction math trick 7

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?

Again, this trick works for numbers with digits 2 and above.

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