**3-digit numbers**

3-digit numbers are made up of hundreds, tens and ones.

For example, 865, 201 etc.

**Mentally**

‘Mentally’ means working out questions in your head without writing them down.

There are different methods of adding and subtracting numbers mentally.

**Partitioning numbers**

214 + 356

To add these two numbers, we can partition the numbers according to the value of the digits and then add.

214 + 356 = 570

**Compensation method**

In the compensation method, we add or subtract an easier closer number and then adjust the answer at the end. For example, adding 10 instead of 9 and taking away 1.

**Example 1:** 423 + 9

**Step 1**: Add 10 to 423

423 + 10 = 433

**Step 2: **Take away 1 from 433

433 – 1 = 432

Hence, 423 + 9 = 432

**Example 2: **756 – 195

**Step 1: **Subtract 200 as 200 is an easier closer number to 195

756 – 200 = 556

**Step 2: **Add 5 to the answer to adjust the subtraction of the extra 5 in step 1.

556 + 5 = 561

Hence, 756 – 195 = 561

**Let's try a question.**

At the start, there were 409 people at the party. Nine people arrived after a few hours. In total, how many people attended the party?

Number of people at the start of the party = 409

Number of people who arrived after a few hours = 9

Total number of people who attended the party = 409 + 9

We can firstly add 10 to 409 and then take away 1.

409 + 10 = 419

419 - 1 = 418

**Total number of people who attended the party = 418**

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