# 2.NBT.B.8*

Mentally add 10 or 100 to a given number 100–900, and mentally subtract 10 or 100 from a given number 100–900.

*TNCore Focus Standard

## Summary:

This standard asks students to be able to think of 10 and 100 not just as generic numbers that can be added or subtracted using the standard algorithm, but as bundles of ten or a hundred that can easily be added to, or removed from, the place-value representation of a number. For example, if we want to add or subtract 10, we can do so simply by adjusting the number of tens in the number, unless doing so takes us over 9 tens or under 0 tens. A similar idea applies when we add or subtract 100.

## Understanding the Standard:

• The use of a hundreds board is especially helpful in teaching this particular standard when adding or subtracting 10. This visual aid allows students to see how numbers increase and decrease from one row to another. The hundreds board also allows the teacher to guide students to explore and identify the patterns in these rows. From here, students can begin to demonstrate how to mentally add and subtract 10 through the movement from one row of numbers to the next. This will reinforce knowledge students are expected to have developed in Grade 1.
• Base ten models also provide an excellent venue for students to physically manipulate the adding and subtracting of tens and hundreds. This concrete demonstration is another tool in explaining how adding and subtracting 10 changes only the number of tens, but not the number of ones. It also explains that adding and subtracting 100 changes only the number of hundreds, and not the number of tens and ones. Base-ten blocks can also help students figure out what to do when adding or subtracting 10 takes them over a "threshold of hundreds", for example, when going from 694 to 704, or vice versa. From there, students should focus on mental strategies for this standard.

## Questions to Focus Instruction:

• Can students add 10 or 100 to a number by increasing the number of tens or 100s  in the number by one and regrouping if necessary?
• Can students subtract 10 or 100 from a number by decreasing the number of  tens or 100s in the number by one and regrouping if necessary?
• Can students explain that adding/subtracting 10 causes an increase/decrease in the number of tens, but not in the number of ones?
• Can students explain that adding/subtracting 100 causes an increase/decrease in the number of hundreds, but not in the number of tens nor in the number of ones?
• Can students mentally add and subtract 10 and 100 from a given number in the range 100-900?
• Can students explain the reasoning behind their strategy?

## Skills

Prior to: Students can mentally add and subtract 10 while explaining their reasoning.  See 1.NBT.C.5.

At Grade Level: Students will build upon their previous knowledge to mentally add and subtract 10 or 100 to/from a three-digit number.

Moving Beyond:
Students will add to their knowledge base by developing mental strategies for addition. For example, compute 539 + 204 by first adding the hundreds(539, 639, 739) then by adding the ones (739, 740, 741, 742, 743)