Yes, he can write it out vertically. You don't have to do all problems mentally. And there is more than one mental process on it.Peter wrote:Hello. I was wondering if anyone can tell me if I am doing this correctly. Everything is pretty self explanatory but we came across one subtraction problem I was not too sure off. It is found in 2B workbook pg. 21 4. (a) 539-70. I was having him take 539 and breaking it down using that triangle symbol (^). We put 530 on one side and 9 on the other. We then mentally counted backwards and subtracted 530 from 70. We then subtracted mentally by counting backwards the remaining ones (9).
I hope someone can understand this! I am not good at explaining things very well. Thank goodness my son is a natural at math. He came up with the right answer. My concern is that it wiped him out mentally after subtracting the 530 from 70. He started to moan and groan. Any thoughts or another way to do this? He wanted to just write it out vertically. I just don't think I should allow him to do this because it defeats the whole purpose of the "mental math thing". Any advice would be greatly appreciated. Thank you!
My oldest would have done mentally but it's exercise 9. so, my oldest daughter's method wouldn't have been taught.
In my middle daughter's workbook, she did not mentally do any in section 4 on p. 21. It turns out she didn't stack them though. She did it horizontally with regrouping -- just like you would with vertical, but she did it horizontally.
I definitely did not make her do that section mentally without help and modeling.
If you want to do it mentally, think of it as taking out groups of tens from the 39, not from 30. Jump back in segments.... do it in 40 then 30
modeled this out loud
first mentally take out 40
that gets you to 499
now you have 30 to go.
first take out 50
take 40 more
so don't take it from the tens' and then units
Then, if your child catches the pattern, continue to do them out loud together in that fashion. 353-70 becomes: 353 minus 50 = 303, go back 2 more rounds of 10's and you get 293, 283.
otherwise, it's ok to stack
I'm thinking I probably modeled some of this for my middle daughter by writing those on the dry erase board.
509 (giggling because I had to edit this number in, I got distractedwhile typing)
and then she'd get stuck and I'd just say
then prompt her for 489
so, if you want to practice it that way, that works too. Those lessons have been taught to this point.