4.8.5 Human Calculator
Now, let’s look at a quick way to attack division by nine.
Activity: The Human Calculator
Watch this short video to discover the power of your brain:
Now see how quickly you can perform these calculations in your head.
(a)
Hint
Comment
Write down the first number of the dividend, then use addition. Watch out for carries!

Answer
(a)
Thus, .
(b)
Answer
(b)
Thus, .
(c) (This one is tough—you might need to write the shortcut out!)

Answer
Thus, .
Aren’t you impressed by the power of your brain? You should be! Of course, the real math is understanding why this trick works, but you’ll have to study much more math to find that out.
Now you have finished the optional activities as well it is time to move onto Unit 5 [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] and continue to work with numbers. To be precise, you will work with various units of measurement and negative numbers. See you there and keep going!
4.8.4 Here’s the Trick!