Mental Addition and Subtraction of 10 and 100

2.NBT.B.8: Mental Addition and Subtraction of 10 and 100

The Common Core State Standards for Mathematics require that students in 2nd Grade be able to mentally add or subtract 10 or 100 to any number from 100 to 900. This skill is important for developing fluency with numbers and understanding the relationships between numbers. In this article, we will explore the concepts of mental addition and subtraction of 10 and 100, and provide examples of word problems to help students practice these skills.

Mental Addition of 10

To mentally add 10 to a given number, you can follow these steps:

1. Look at the number and identify the ones place.
2. If the ones place is 0-6, add 10 to it.
3. If the ones place is 7-9, add 10 to it and carry 1 over to the tens place.
4. Add the carried over 1 to the tens place and any other digits there.

Example: Add 10 to 382.

1. The ones place is 2, so add 10 to it: 2 + 10 = 12.
2. Since there is room in the ones place, no further action is needed.
3. The new number is 392.

Mental Addition of 100

To mentally add 100 to a given number, you can follow these steps:

1. Look at the number and identify the hundreds place.
2. If the hundreds place is 0, add 100 to it.
3. If the hundreds place is 1-9, add 100 to it and carry 1 over to the thousands place.
4. Add the carried over 1 to the thousands place and any other digits there.

Example: Add 100 to 382.

1. The hundreds place is 8, so add 100 to it: 8 + 100 = 108.
2. Since there is room in the hundreds place, no further action is needed.
3. The new number is 108.

Mental Subtraction of 10

To mentally subtract 10 from a given number, you can follow these steps:

1. Look at the number and identify the ones place.
2. If the ones place is 0, subtract 10 from it and carry 10 over to the tens place.
3. If the ones place is 1-9, subtract 10 from it.
4. Subtract the carried over 10 from the tens place and any other digits there.

Example: Subtract 10 from 382.

1. The ones place is 2, so subtract 10 from it: 2 - 10 = -8.
2. Since there is no room in the ones place, carry 10 over to the tens place.
3. The new number is 372.

Mental Subtraction of 100

To mentally subtract 100 from a given number, you can follow these steps:

1. Look at the number and identify the hundreds place.
2. If the hundreds place is 10-99, subtract 100 from it and carry 100 over to the thousands place.
3. Subtract the carried over 100 from the thousands place and any other digits there.

Example: Subtract 100 from 382.

1. The hundreds place is 8, so subtract 100 from it: 8 - 100 = -92.
2. Since there is room in the hundreds place, no further action is needed.
3. The new number is -2.

Drawing Word Problems

Drawing word problems can help students visualize the problem and understand the relationships between numbers. For example, to solve the problem "Subtract 10 from 382," you could draw a number line with arrows pointing to the left for subtraction. Label the number line with the given number, 382, and the target number, 272. Then, draw an arrow from 382 to 372, representing the subtraction of 10.

To help students practice mental addition and subtraction of 10 and 100, you can create word problems involving these operations. For example:

1. Add 10 to 479.
2. Subtract 10 from 865.
3. Add 100 to 623.
4. Subtract 100 from 789.

By practicing these mental addition and subtraction skills, students will develop fluency with numbers and a better understanding of the relationships between numbers.