Hey guys!!! Want to know how to do subtraction like a calculator.?? I will tell you how to subtract big numbers like a pro!!

This is known as “Mental Subtraction”

The most important idea with mental subtraction problems is the strategy of “oversubtracting“. Let’s see this example:

83 − 9 = 73 + 1 = 74

Now let’s see another.

64 – 28 = 34 + 2 = 36

Did you notice a pattern??

When subtracting numbers with two or more digits, the key idea is
to use “complements.” (You’ll compliment me later for this.) The complement of a number is its distance to the next-highest round number. With one-digit numbers, this is the distance to 10. (For example, the complement of 9 is 1.) For two-digit numbers, this is the distance to 100. Look at the following pairs of numbers that add to 100. What do you notice?

We say that the complement of 87 is 13, the complement of 75 is 25,
and so on. Conversely, the complement of 13 is 87 and the complement
of 25 is 75. The exception is when the numbers end in 0. For example, the complement of 80 is 20

Now let’s apply this strategy!!! Here’s the first question.

4678 – 982 = ?

I have written this left to right to avoid you take out pen and paper!! Now complementary of 982 is 1000. First, we subtract 982 from 1000 i.e., 18 and then 1000 from 4678 i.e., 3678 and then we will add both the results.

4678 – 982 = 3678 + 18 = 3696

Look how easy it becomes. Let’s try another one! A big one!!

987268 – 98628 = ?

Now again we will try our strategy. The complementary of 98628 is 100000. So first we will subtract 98628 from 100000 that comes out to be 1372. Second, we will subtract 100000 from 987268 i.e, 887268 and finally we will add both the results.

987268 – 98628 = 887268 + 1372 = 888640

Notice that the addition problem is especially easy because there are no “carries” involved.

Hope you like my post! Let me know in the comments and for making your mathematical calculations easier, stay tuned to MathCompt!

Reference : The Magic of Math by Arthur Benjamin