# 1.NBT.C.5

Given a two-digit number, mentally find 10 more or 10 less than the number, without having to count; explain the reasoning used.

Click here to learn about the process used to unpack the standards and create learning targets.

## Summary:

This standard asks students to compute (quickly and easily) throughout the range of numbers using the strategy of mental mathematics. Students’ knowledge of the structure of tens will aid in their reasoning and demonstration of mentally adding and subtracting 10. Given a two-digit number, students should recognize that, when adding or subtracting 10, the only digit that is affected is the digit in the tens place. Further, their original understanding that addition means to “add on” will help them to identify that by adding 10, there will be an increase in the number of tens, but the number of ones will stay the same. Additionally, students must understand that subtraction means to “take away” and therefore, when subtracting 10, there will be a decrease in the a number of tens in a two-digit number while the a number of ones will stay the same.

## Understanding the Standards:

• The use of a hundreds board is especially helpful in teaching this particular standard. 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.
• Base ten models also provide an excellent venue for students to physically manipulate the adding and subtracting of tens. 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.

## Specific Questions to Focus Instruction:

• Can students explain that adding 10 indicates that there will be an increase in the amount of tens they now have, but the number of ones will stay the same?
• Can students explain that subtracting 10 indicates that there will be a decrease in the number of tens they now have, but the quantity of ones will stay the same?
• Can students mentally add or subtract 10 from a given two-digit number?
• Can students explain their method for getting their solution and the reasoning behind it?

## Skills

Prior to: Students can count fluently to 100 and can compare two-digit numbers accurately. (1.NBT.B.3 and 1.NBT.C.4)

At Grade Level: Students can mentally add and subtract 10 using a variety of strategies while explaining their reasoning.

Moving Beyond:
Students will build upon their previous knowledge in order to mentally add and subtract 100. Go to 2.NBT.B.8 to see the progression of related skills.