I'm so sorry I am just getting back to you, Jessica, but you had some great responses already!
Pjdobro explained it just like I would have. That is what we do as well. The hands-on math lesson from Unit 27, Day 4 teaches that strategy of counting up to mentally find the answer and takes it one step further. Here is that problem and the hands-on lesson for it, and the taking it one step further part is bolded...
*If you have $3.30 - $0.99 = ____, you can think to yourself, $0.99 is $0.01 away from $1.
I can think of $3.30 as $2.30 + $1.00. I can easily subtract $0.99 from the $1.00 part to get 1 cent left. Then, that leaves me with the $2.30 and the 1 cent left, which is $2.31.
So, by thinking of the biggest number $3.30 as being the same as $2.30 + $1, makes it easy to subtract the buck and then add the penny. Doing multiple problems like that on the marker board with your ds if he's having a tough time with it would be a good idea. That is a tougher concept, but learning that strategy sure makes it easier to do once the strategy is in place. So, for instance, here are a few problems to get your ds practicing it...
$4.20 - $0.99 = ___
Think of $4.20 as $3.20 + $1
Take way the $1 and get $3.20
Add the penny since you really needed to take away 99 cents, and the answer is $3.21
$5.50 - $.98 = ____
Think of $5.50 as $4.50 and $1
Take away the $1 and get $4.50
Add the two pennies since you really needed to take away 98 cents, and the answer is $4.52
$10 - $2.95 = ____
Think of $10 as $7 and $3
Take away the $3 and get $7
Add the nickel since you really needed to take away 95 cents, and the answer is $7.05
Now, if this is all kind of confusing, some dc do better to begin with just counting up. So, for the previous problem, you'd count up from $2.95 to $3 first, remembering that took you 5 cents. Then, count up from $3 to $10, which took $7. So, together that's $7.05.
Likewise, this works in adding.
$3.85 + $2.05 = ____
Think of it as $3.85 + $2 = $5.85
Since we were really needing to add 5 cents more, the answer is $5.90
$4.55 + $0.99 = ______
Think of it as $4.55 + $1 = $5.55
Since we added one more cent than we should have by adding $1 instead of 99 cents, take away 1 cent and the answer is $5.54.
This kind of playing with numbers can be used in this problem too:
369 + 631
Take the 1 from the 631 and add it to the '69' part of 369 to get '70'. Add the '70' and '30' to get '100'. Now just add 300 + 600 + 100
Essentially, that's what we'd be doing on our good old scratch paper, but thinking of it this way makes it possible to do it quickly and mentally. I used problems from the Review you mentioned to try to help explain this.
I will say that when we started this in Singapore, I
didn't get it and neither did my ds at first, BUT, then with doing it awhile,
HE got it and I was still limping along with it. It is like I am learning mental math right along with him.
I so looooooooong to reach for scratch paper and just do it the old way, and likewise, I was tempted to just show him how to do it on paper instead and tell him to forget that other way of doing it. I have learned the hard way that it is better NOT to give into temptations to show "my old way" of doing it to him. It confuses him more than anything. My oldest ds in 4B now uses this strategy ALL OF THE TIME on his own. I am just getting how to do it myself. I've been trying to make myself use the Singapore way of doing things, and I am needing less and less scratch paper.
That being said, if your ds wants to use scratch paper or hands-on objects for awhile, I'd let him, but then I'd reteach the HOD hands-on lessons and the Singapore strategies they match with until he understands it better. It is important to know that eventually Singapore mixes problems that this strategy works with along with problems that do probably just need to be written out the long way - BUT, by then our d can spot the difference and use mental math as much as possible. In fact, my ds has this strange way of carrying and borrowing that doesn't require him to write out the entire problem lined up perfectly at all. I won't explain that here - not sure I can, but let me say my ds learned his facts later than most dc doing other math programs, but that was so worth it as the mental process was learned along with the facts which benefited him more in the long run. HTH!
In Christ,
Julie
Now, granted, I am NOT a math person and I don't think it is really his strength either, but it was frustrating me as well as him. 