Singapore 2B Exercise 38, subtracting $0.99

[momto2boys]

Does anyone have any tips on teaching how to add 1c to $0.99 to get to a dollar to subtract? He keeps saying"I don't get it" when I explain it from the book.It's Unit 27 day 4 from BHFHG. Thanks for your help!

Blessings,
Nicole

[MelInKansas]

My only suggestion is to use manipulatives and try with 10's first instead of 99c and $1 since that would be harder to represent and visualize with manipulatives. So use some kind of counters, or dimes and pennies, that represent 10's and 1s and talk about having 76 + 9 for example or 76 - 9. Then say "well an 'easier' way to do this is to use a 10 instead of 9, since 9 is ALMOST 10 we will add one more and make it 10. Then the problem becomes 76 - 10. That's easy to do in your head right? Just take one away from the 10's. But you added in one extra to make that 10. Then when you subtracted you subtracted one more than you needed to, so we have to add that one back in now. So you keep track of any extras. Since you subtracted one extra, you need to add one back in and your answer would be 67." Do this several times and have him do it on his own. Then talk about 99c and $1 and explain that it's exactly the same way. Sometimes my kids need the manipulatives and the visualisation of it.

[momto2boys]

Thanks! I wound up getting out money and showing him. He got them all right, but it I still don't think that he "understands" it.

[MelInKansas]

I always get confused on whether you should add or subtract the extra one back in when you are done. It's the opposite of whatever the operand in the problem was. So I wouldn't say that I think my children "understand" it or will use this technique a lot right now, but the exposure is good. I have seen my oldest really grow in being able to do the "mental math" by changing things into terms she can work with more easily in her head, and figuring problems out in ways I never would have suggested, but they obviously work for her.