Rounding decimals: how much paint?
To round a decimal to the nearest whole number, look at the number after the decimal point. If it is 0, 1, 2, 3 or 4, we keep the whole number: 3.4 is about 3. If it is 5, 6, 7, 8 or 9, we go up: 3.6 is about 4. Rounding helps us to estimate. The area of a wall is its length times its height. A wall that is 2.9 m high and 4.1 m wide is about 3 m by 4 m. So its area is about 3 × 4 = 12 m². We say about, nearly or around, because it is an estimate. If one can of paint covers 10 m², we need 2 cans, because 12 ÷ 10 = 1.2 and we cannot buy 0.2 of a can.
| Number | Rounded | Why |
|---|---|---|
| 3.4 | 3 | 4 is less than 5 |
| 3.6 | 4 | 6 is 5 or more |
| 7.5 | 8 | 5 goes up |
| 2.1 | 2 | 1 is less than 5 |
The wall is 2.9 metres high and 4.1 metres wide.
Round the numbers. 2.9 is about 3, and 4.1 is about 4.
So the area is about 3 times 4, which is 12 square metres.
One can covers 10 square metres. How many cans do we need?
Two cans, because one can is not enough.
| Height | Width | Rounded | Area |
|---|---|---|---|
| 2.9 m | 4.1 m | 3 m by 4 m | about 12 m² |
Rahma wants to paint a small wall in her room. The wall is 1.8 metres high and 3.2 metres wide. She rounds the numbers: 1.8 is about 2, and 3.2 is about 3. Then she multiplies: 2 times 3 is 6. The area is about 6 square metres. One can covers 8 square metres, so one can is enough.
Estimate the room. With the grown-up, measure a door or a table with a tape or a string and write the length in metres with one decimal. Round it, then estimate the area with a second side. Say: It is about … square metres.
| Height | Width |
|---|---|
| 1.9 m | 3.1 m |