Add and subtract three-digit numbers (don’t remember)

### 1.1. Knowledge to remember

To add and subtract three-digit numbers, do the following:

- Set calculation: Put the digits in the same row and column together.
- Calculation: Perform adjectives from right to left.

### 1.2. Solving Textbook Exercises

**Lesson 1:** Mental arithmetic:

a) 400 + 300 = b) 500 + 40 = c) 100 + 20 + 4 =

700 – 300 = 540 – 40 = 360 + 60 + 7 =

700 – 400 = 540 – 500 = 800 + 10 + 5 =

**Solution guide:**

a) 400 + 300 = 700 b) 500 + 40 = 540 c) 100 + 20 + 4 = 124

700 – 300 = 400 540 – 40 = 500 300 + 60 + 7 = 367

700 – 400 = 300 540 – 500 = 40 800 + 10 + 5 = 815

**Lesson 2:** Calculate then calculate

352 + 416; 732 – 511; 418 + 201; 395 – 44.

**Solution guide:**

\(\frac{{ + \begin{array}{*{20}{c}}

{352}\\

{416}

\end{array}}}{{\,\,\,768}}\) \(\frac{{ – \begin{array}{*{20}{c}}

{732}\\

{511}

\end{array}}}{{{\rm{ }}\,\,\,221}}\) \(\frac{{ + \begin{array}{*{20}{c}}

{418}\\

{201}

\end{array}}}{{{\rm{ \,\,\,619}}}}\) \(\frac{{ – \begin{array}{*{20}{c}}

{395}\\

{{\rm{ }}\,\,44}

\end{array}}}{{{\rm{ \,\,\,351}}}}\)

**Lesson 3:**** **Grade One has 245 students, Grade Two has 32 students less than Grade One. How many students in second grade?

**Solution guide:**

The number of second grade students is:

245 – 32 = 213 (student)

**Lesson 4:** The price of an envelope is 200 dong, the price of a postage stamp is 600 dong more than an envelope. How much does a postage stamp cost?

**Solution guide:**

The price of a postage stamp is:

200 + 600 = 800 (VND)

**Lesson 5:** With three numbers 315, 40, 355 and the signs +, -, =, make the correct calculations:

**Solution guide:**

The following calculations can be made:

315 + 40 = 355 355 – 40 = 315

40 + 315 = 355 355 – 315 = 40

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