Teacher2Teacher |
Q&A #1521 |
From: Loyd
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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