Why eighty is a resting place
Before you read anything
Count to a hundred in tens, out loud. Ten, twenty, thirty — all the way.
Now count to a hundred in sevens. Stop when it stops being easy.
The question module 2 left you with
$85 - 8$ was easy because you stopped at eighty on the way. Counting in tens was easy just now, and counting in sevens was not.
Eighty is restful because it is eight tens and nothing over. That is the whole answer, and everything in this module comes out of it.
Ten is the size we count in
:x nutshell-place-value
What a digit is worth, decided by where it sits. The 4 in 40 is four tens; the 4 in 400 is four hundreds.
A number is not a row of digits. It is a count of tens and a count of ones, written down — which is :place value, and it is the reason $85 - 5$ needs no thought and $85 - 7$ does.
Read it off the bundles
Eight bundles and three loose sticks is eighty-three. Take the three away and the bundles are untouched: eighty. Take four away and you have to open one.
That is the whole difference. Stopping at a ten means stopping where nothing has to be opened.
Check yourself
Six to try
How many tens are there in $50$?
How many tens are there in $40$?
How many tens are there in $70$?
You have $17$ ones. You bundle ten of them into a ten. How many ones are left over?
You have $13$ ones. You bundle ten of them into a ten. How many ones are left over?
You have $11$ ones. You bundle ten of them into a ten. How many ones are left over?
Away from the screen
Show that you have it
The digit, and what it is worth
In $375$, what is the $7$ worth?
In $541$, what is the $4$ worth?