Teacher2Teacher

Q&A #1521

Teachers' Lounge Discussion: Subtraction

T2T || FAQ || Ask T2T || Teachers' Lounge || Browse || Search || T2T Associates || About T2T

View entire discussion
[<< prev] [ next >>]

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.

Post a reply to this message
Post a related public discussion message
Ask Teacher2Teacher a new question


[Privacy Policy] [Terms of Use]

_____________________________________
Math Forum Home || The Math Library || Quick Reference || Math Forum Search
_____________________________________

Teacher2Teacher - T2T ®
© 1994- The Math Forum at NCTM. All rights reserved.
http://mathforum.org/