Practice addition and subtraction

### 1.1. Solving exercises in the textbook Practice pages 160, 161

**Lesson 1 Textbook page 160**

Calculate

\(\begin{array}{l}

a)\frac{2}{3} + \frac{3}{5};\frac{7}{{12}} – \frac{2}{7} + \frac{1}{{12}} ;\frac{{12}}{7} – \frac{5}{{17}} – \frac{4}{{17}}\\

b)578.69 + 281.78; 594.72 + 406.38 – 329.47

\end{array}\)

__Solution guide:__

\(\begin{array}{l}

a)\frac{2}{3} + \frac{3}{5} = \frac{{10}}{{15}} + \frac{9}{{15}} = \frac{{19} }{{15}}\\

\frac{7}{{12}} – \frac{2}{7} + \frac{1}{{12}} = \frac{{49}}{{84}} – \frac{{24} }{{84}} + \frac{7}{{84}}\\

= \frac{{25}}{{84}} + \frac{7}{{84}} = \frac{{32}}{{84}} = \frac{8}{{21}}\\

\frac{{12}}{{17}} – \frac{5}{{17}} – \frac{4}{{17}} = \frac{7}{{17}} – \frac{4 }{{17}} = \frac{3}{{17}}

\end{array}\)

\(\begin{array}{l}

b)578.69 + 281.78 = 860.47\\

594.72 + 406.38 – 329.47 = 1001.1 – 329.47 = 671.63

\end{array}\)

Lesson 2 Textbook page 160

Calculate by the most convenient way:

\(\begin{array}{l}

a)\frac{7}{{11}} + \frac{3}{4} + \frac{4}{{11}} + \frac{1}{4}\\

b)\frac{{72}}{{99}} – \frac{{28}}{{99}} – \frac{{14}}{{99}}\\

c)69.78 + 35.97 + 30.22;\\

d)83.45 – 30.98 – 42.47.

\end{array}\)

__Solution guide:__

\(a)\frac{7}{{11}} + \frac{3}{4} + \frac{4}{{11}} + \frac{1}{4}\)

\(\begin{array}{l}

= \left( {\frac{7}{{11}} + \frac{4}{{11}}} \right) + \left( {\frac{3}{4} + \frac{1}{ 4}} \right)\\

= \frac{{11}}{{11}} + \frac{4}{4} = 1 + 1 = 2

\end{array}\)

\(b)\frac{{72}}{{99}} – \frac{{28}}{{99}} – \frac{{14}}{{99}}\)

\(\begin{array}{l}

= \frac{{72}}{{99}} – \left( {\frac{{28}}{{99}} + \frac{{14}}{{99}}} \right)\\

= \frac{{72}}{{99}} – \frac{{42}}{{99}} = \frac{{30}}{{99}} = \frac{{10}}{{33 }}

\end{array}\)

\(\begin{array}{l}

c)69.78 + 35.97 + 30.22\\

= (69.78 + 30.22) + 35.97\\

= 100 + 35.97 = 135.97\\

d)83.45 – 30.98 – 42.47\\

= 83.45 – (30.98 + 42.47)\\

= 83.45 – 73.45 = 10

\end{array}\)

Lesson 3 Textbook page 161

A family of workers uses their monthly salary as follows: \(\frac{3}{5}\) the salary to pay for the family’s meals and children’s education, \(\frac{1 }{4}\) salary to pay rent and other expenses, the rest is for savings.

a) What percentage of salary does the family save each month?

b) If the salary is 4 000 000 VND per month, how much money does the family save each month?

__Solution guide:__

a) Consider the salary of that family as 1 unit.

The fraction that shows the share of the salary that the family spends for the month is:

\(\frac{3}{5} + \frac{1}{4} = \frac{{17}}{{20}}\) (wage amount)

The percentage of that family salary saved is:

\(1 – \frac{{17}}{{20}} = \frac{3}{{20}}\) (wage amount)

\(\frac{3}{{20}} = \frac{{15}}{{100}} = 15{\rm{\% }}\)

b) The amount the family has saved is:

4000000 : 100 × 15 = 600000 (VND)

Answer: a) 15% ;

b) 600000 VND.

.

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