Add, subtract, multiply, and divide decimals (grade 5) · lesson

Line up the points

💡 To add or subtract decimals, line up the decimal points so each place matches.

Tenths go under tenths and hundredths under hundredths. Lining up the points does this.

Fill empty places with zeros. 4.3 is 4.30, and 7 is 7.00.

🧠 Let's work it out

Find 7 − 2.48.

What do you think comes next?

💡 Multiply as if there were no points, then give the answer as many decimal places as both numbers have together.

To find 0.3 × 0.7, first find 3 × 7 = 21.

0.3 has 1 decimal place and 0.7 has 1. That is 2 in all, so the answer is 0.21.

If the answer ends in a 0 after the point, you can drop it. 0.50 is the same as 0.5.

🧠 Let's work it out

Find 1.6 × 0.3.

What do you think comes next?

💡 Money is decimals: dollars on the left of the point and cents on the right.

To find the cost of many items, multiply the price by how many. To find change, take the cost from what you paid.

If a story has two steps, find the total cost first. Then find the change.

🧠 Let's work it out

Notebooks cost $2.15. You buy 4 and pay with $20. How much change do you get?

What do you think comes next?

💡 Multiply both numbers by the same power of 10 until you divide by a whole number.

Multiplying both numbers by 10 does not change the answer. It just makes the numbers easier.

Use 10 if the divisor has 1 decimal place. Use 100 if it has 2.

🧠 Let's work it out

Find 2.25 ÷ 0.75.

What do you think comes next?