Friday, August 10, 2012

Using Money to Teach Borrowing in Subtraction

Scholar loves math.
I guess it's in his blood -
as there are a large number of mathematicians
in his lineage.

He also loves to play with money.

But the concept of carry and borrow with 3 digits was confusing him.

In s sudden inspiration,
I pulled out the play money.

I gave him pennies (ones)
dimes (tens)
dollars (hundreds)

He grinned and quipped,
"If you give me the ten dollars, I'll have thousands!"

I grinned back,
he understood the substitution of place value.

Usually we used our counting sticks and counting blocks.

"You have to start with the pennies,"
I told him.

We did the first problem together.

We did it the wrong way,
so he could see why it wouldn't work.

Then we did it the right way.

So his problem was 356 - 149.

He put down 3 dollars,
5 dimes
and 6 pennies.

Then he put on his funny voice.

"I need 9 pennies!"
"I don't have 9 pennies, there are only 6 of us"
"Come here dime, go sit down with the pennies."
"Wait, you don't match .... I'm going to put you up here and take 10 pennies."
"NOW there are 9 pennies."

He took 9 pennies from the pile and wrote down 7.

"I need 4 dimes"
"But that is ALL of us"
"Too Bad"

He scooped up the 4 dimes and put them back into the big pile,
and wrote down 0.

"Hmmm ..... I only need 1 dollar."

He took the dollar, stared at the 2 left,
laughed and wrote down the 2.

Then on to the next problem:  825 - 207.

"207!" he exclaimed.
"Hey, is the answer always in the next problem?"

He set out 8 dollars
2 dimes
and 5 pennies.

Then he did the whole comical borrowing routine all over again.

The third time through ...
he opened a store and started selling imaginary
candies to people.


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