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Topic

Topic Progress:

Let’s take one more look at the lunch order, noticing how the numbers were added together.

All three items (sandwich, water, tax) were priced in dollars and cents, so we lined up the dollars under the dollars and the cents under the cents, with the decimal points lined up between them. Then we just added each column, as if we were adding whole numbers. By lining up decimals this way, we can add or subtract the corresponding place values just as we did with whole numbers.

Steps to take:

1. Write the numbers vertically so the decimal points line up.
2. Use zeros as place holders, as needed.
3. Add or subtract the numbers as if they were whole numbers. Then place the decimal in the answer under the decimal points in the given numbers.

### Example: Add: 3.7 + 12.4

Solution
  Write the numbers vertically so the decimal points line up.  Place holders are not needed since both numbers have the same number of decimal places. Add the numbers as if they were whole numbers. Then place the decimal in the answer under the decimal points in the given numbers. 

17.6

33.11

Watch this example to see how Sal Kahn handles addition with decimals:

## Subtracting Decimals

How much change would you get if you handed the cashier a $20 bill for a$14.65 purchase? We will show the steps to calculate this in the next example.

### Example: Subtract: 20 - 14.65

Solution
 20−14.6520−14.65 Write the numbers vertically so the decimal points line up. Remember 20 is a whole number, so place the decimal point after the 0. Place two zeros after the decimal point in 20, as place holders so that both numbers have two decimal places. Subtract the numbers as if they were whole numbers. Then place the decimal in the answer under the decimal points in the given numbers.

0.42

### Subtract: 50−37.42

12.58

Watch this example to see how Sal Kahn handles subtracting with decimals: