If we follow the convetional rules of addition, we will start of by adding the right numbers which is 8 + 2 = 10. So we write 0 and bring 1 to the second right number. Then we add again 9 + 7 + 1 = 17 … and so on.

But take a look at the number 9998. This number is close to 10,000. We can say that:

9998 = 10,000 – 2

Therefore its a lot easier to add 10,000 to 6572 and then substract the result by 2.

Easy ? This method can be extended to adding multiple numbers which is close to a whole number. Take for example:

We can calculate by first finding how much each number need to add to 100. We found that it is 7 , 3 , 2 , 5 respectively from up to down. 7 + 3 + 2 + 5 = 17. Now we can calculate the question above as 400 – 17 = 383.

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