DS showed me a trick he said he made up for subtracting seven from double digit numbers. I don't know if he actually made this up or saw it someplace, but he insists he was the one who thought of it.
Can anyone tell me if they've seen this type of computation before and what it's called?
He will take an equation like 15-7= and add three to the seven making it a ten then cross out the one in the fifteen, making it a zero, and add three to the five to get the answer of 8.
I didn't understand what he was doing as he was muttering under his breath all this. I said, "What are you doing?" He explained and being the math dunce I am, I didn't understand his first three explanations. Finally I wrote down equations ranging from 10-7 through 19-7 and he showed me on paper. It works for all of them. When I asked DS about an equation like 23-7, he said he'd add three to the 7 and cross off the two, make it a one and add three to the three in 23 to make a six. So then bring down the one next to the six and the answer is 16.
What is this?
He also has tricks for subtracting 8's and 6's. He just adds either a 2 or 4 respectively and completes the equation just as he does for the 7's.
