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Q&A #1521

Teachers' Lounge Discussion: Subtraction

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From: Loyd <Loydlin@aol.com>
To: Teacher2Teacher Public Discussion
Date: 2003112115:50:16
Subject: Re: Subtraction; tens complement

I was tutoring a second grader one time and he told me how he
subtracted 8 and 9 from two digit numbers; he learned the method in
his school.  

EX:  15-8.  Since 8 has two holes, add two to 5 and the answer is 7. 
9 has one hole and 5 + 1 =6 so the answer is 6.  

What the student is doing is finding the tens complement; a method
that is often used in teaching binary addition and subtraction.  He
ignored the fact that he is also subtracting ten as follows:

15 - 8 = 15+2-10 = 17 - 10 = 7
15 - 9 = 15 + 1 - 10 = 16 - 10 = 6
15 - 4 = 15 + 6 - 10 = 11


I am not recommending this method for all students, but this works for
some students.  





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