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# Subtracting in Two’s Compliment Binary watch

1. Hello I’m struggling in how to do this, there are plenty of tutorials online but they aren’t particularly helpful. For example, if I have 1010 (10) minus 0101 (5) how to get to the answer? I have realised that when doing:
1010
-0101
Then by borrowing from the left in 1010 I end up with 0101 which equals 5, is this a coincidence that by borrowing the left I get the correct answer I want?

Thank you for your help
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2. (Original post by ManDilne)
Hello I’m struggling in how to do this, there are plenty of tutorials online but they aren’t particularly helpful. For example, if I have 1010 (10) minus 0101 (5) how to get to the answer? I have realised that when doing:
1010
-0101
Then by borrowing from the left in 1010 I end up with 0101 which equals 5, is this a coincidence that by borrowing the left I get the correct answer I want?

Thank you for your help
Although I'm not familiar with this, it looks like you borrow from the left.

Start with the 4th digits, 0-1 means you borrow 1 from the left digit to make it 10-1, which is 1.

3rd digits are now 0-0 = 0.

Second, 0-1 again = 1 with borrowing from left.

First digits are now 0-0 = 0.

So you end up with 0101.

So you work from right to left and borrow as necessary.

http://sandbox.mc.edu/~bennet/cs110/pm/sub.html

Extra practice here

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