Math Monkey

How to add (or subtract) large numbers in your head

Math Monkey

The monkey’s name is Robert. Photo by Maribel.

Being able to add large numbers in your head can be pretty useful sometimes. Here’s how I do it:

Let’s take two relatively large numbers:

4589 +
1578

The first thing I do is round them up (or down) to the nearest hundred (could be thousand, depends how large the numbers are).

4600 +
1600

This is much easier to add.

6200

Now, since we rounded the first number up we need to subtract what we added. It was 11. The second number we also rounded up, so we need to subtract 22. In total we need to take out 33 from that. That’s another easy one:

200 – 33 = 167

And the result is 6167.

Subtraction is very similar:

2375 –
7351

That’s 2400 – 7400, pretty easy, right?

-5000.
The first number we rounded up, so we add 25. The second number we also rounded up, but we have to take the sign into account. We are going to add -49 to 25. That’s -24. That result we then subtract from the -5000. Again, keep the sign in mind.

-5000 – (-24) = -5000 + 24 = -4976

Ok, I admit that working with negative numbers is trickier, in the end it’s a matter of keeping the sign straight. You can turn the numbers around, 7351 – 2375, to make it a bit easier, just remember that the result will be negative.

In essence, both operations are the same: round up, add, add what you rounded up (take the sign into account) and subtract that from the result. A bit of practice and you’ll master it in no time.

What math tricks do you use?