I have been reading a book recently about how to calculate fast!! The title of the book is
The Trachtenberg Speed System of Basic Mathematics.
It's real fun and cool. Methods to do speed multiplications are taught. Adding a long list of numbers, multiplying by multiple digit like 1646461314 x 46146749. If you master the methods, you can easily get the answer without showing any workings in less than a minute. Of course, I'm new and no expert, so, it takes me definitely more than a minute to get the answer! Haha..
Here's a little bit from the book. How to multiply any number by 11? It's the first trick taught in the book and it's one of the easiest. Anyone who knows how to add, can do it! Yes, so we are saying our children who are maybe 7 years old, knows how to add single digits, can get this answer correct.
Example 34261 x 11 = 376871. The answer can be gotten in less than half a minute.
HOW?!? Let's work it step by step from left to right of 34261.
Step 1: Write 1, under the 1 of 34261.
Like this;
34261
1
Step 2: Write 7 under the digit 6. This is gotten by adding 6 and 1. (6+1 = 7)
Like this:
34261
71
Step 3: Write 8 under the digit 2. This is from 2+6=8
Like this:
34261
871
Step 4: Write 6 under the digit 4. This is from 4+2=6
Like this:
34261
6871
Step 5: Write 7 under the digit 3. This is from 3+4=7
Like this:
34261
76871
Step 6: Finally, write 3, in front of 7. This is from 0+3=3.
Like this:
034261
376871 << Answer. Check with your calculator.
And also try out other numbers... Longer numbers.. You are better than the calculator now, coz some calculators can only do up to 12 digits. You can do up to .... infinite number of digits. :D Like 31464631647964131646413136469463164794849496416494794161646 x 11. So, even if it is like 1 metre long, you won't have a problem getting the correct answer.
Have fun and you can try getting this book!! More fun stuff in it. I'm trying to remember all the rules. Not sure why I'm reading it...though I'm not a mathematics teacher. And definitely not taking any Mathematics examination in the near future. Maybe one day I can start teaching my nieces and nephews the tricks in this book!! And they'll definitely be more interested in Mathematics.

Click on the book to for more details. 