Teacher2Teacher 
Q&A #1521 
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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: 158. 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+210 = 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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