Workplace Math and PLAR Prep

Addition and Subtraction

Addition

The sign for addition is + and it is called a plus sign. This sign tells us to add.

The answer is called a sum or a total.

Let’s take 456 + 289.

1. Line up the place values.

2. Add each column starting from the right, adding the ones first. 6 plus 9 is 15. Put the 5 in the ones column and then carry over the 1 to the next column.

3. Add the tens column. 5 plus 8 plus 1 is 14. Put the 4 in the tens column and carry over the one to the next column.

4. Add the hundreds column. 4 plus 2 plus 1 is 7.

5. Your answer is 745

Estimating Sums

A quick way to estimate the sum of two numbers is to round each number and then add the rounded numbers. This probably won't be the exact answer but it may be close enough for some purposes.

How to estimate a sum by rounding.

  1. Round each number that will be added

  2. Add the rounded numbers.

Some uses of rounding are:

  • Checking to see if you have enough money to buy what you want.

  • Getting a rough idea of the correct answer to a problem


For example: Estimate 235 + 585

1. Round each number. 200 + 600

2. Add. 800

3. Actual answer: 820


How to Improve the Estimate.

1. Round each term that will be added.

2. Add the rounded numbers.

3. If both are rounded down or both rounded up see if the amount of rounding is more than 50. If it is, add or subtract 100 to the estimate.

4. If one number is rounded down and the other is rounded up a closer estimate will not be produced by this method.


For example: Estimate 445 + 735

1. Round each term. 400 + 700

2. Add. 1100

3. Rounded down by more than 50 so add 100. 1100 + 100

4. Rounded answer: 1200

5. Actual answer: 1180

Subtraction

The sign for subtraction is − and it is called a minus sign. Subtraction is taking away one number from another and that is why it is called the minus or takeaway sign.

You can’t take a bigger number away from a smaller number so, sometimes you have to regroup numbers to subtract. Regroup means you borrow from the next place value and add it on to a smaller number so then you have a bigger number to subtract the smaller number from.

For example: 625 – 248

1. Write this problem in a line going down.

2. Subtract the ones column. You can’t take 8 away from 5 so you have to regroup and borrow 1 ten (10) from the tens place.

Now you have 15 – 8 = 7

3. Now you subtract the tens column. You borrowed 1 ten so you have 1 ten left. You can’t take 4 away from 1 so you have to borrow from the

hundreds place. Now you have 11 tens. 11 – 4 = 7

4. You had to borrow 1 hundred from the hundreds place so now you have 5 hundreds left. 5 – 2 = 3 hundreds

5. 625 – 248 = 377

Now let’s take 900 – 548

1. Write this problem in a line going down.

Try it yourself, then click on the arrow to see the answer ---->

2. Subtract the ones column. You can’t take a 8 away from 0 so you have to regroup and borrow 1 ten from the tens place. But there are 0 tens, so you have to borrow 1 hundred from the hundred place.

3. Now you have 10 tens so you can borrow one ten.

4. Subtract the ones column: 10 – 8 = 2.

Now subtract the tens column: 9 – 4 = 5

Now subtract the hundreds column: 8 – 5 = 3

5. 900 – 548 = 352

Remember: Follow the same steps for any subtraction problem.

To check subtraction problems you can add the answer to the number you

subtracted and you should get the other number.

For example: 352 + 548 = 900

Estimating Differences

We use the same method for estimating differences as we do for adding sums. Round each number and then subtract the rounded numbers. This probably won't be the exact answer but it may be close enough for some purposes.

How to estimate a difference by rounding.

1. Round each term that will be subtracted.

2. Subtract the rounded numbers.

For example: Estimate 855 – 385

1. Round each term. 900 – 400

2. Subtract. 500

3. Actual answer: 470

For more examples watch the video below:

Want more practice?

See the worksheets below to print or view online

Math #2 Adding and Subracting.pdf