Sam's dad spends 1/2 of his salary on rent and 2/5 on petrol. What fraction is left spend on food and other necessities for the month?

$$\text{Total spent} = \frac{1}{2} + \frac{2}{5}$$

To add these fractions, find a common denominator, which is 10.

$$\frac{1}{2} + \frac{2}{5} = \frac{1 \times 5}{2 \times 5} + \frac{2 \times 2}{5 \times 2} = \frac{5}{10} + \frac{4}{10} = \frac{5+4}{10} = \frac{9}{10}$$

The total salary is represented by 1.

$$\text{Fraction left} = 1 - \frac{9}{10} = \frac{10}{10} - \frac{9}{10} = \frac{1}{10}$$

The fraction left for food and other necessities is $\boxed{\frac{1}{10}}$.

3.1.1
To arrange the decimal numbers $1,008; 1,8; 1,08; 1,75; 1,88$ in descending order, compare them from largest to smallest.
Comparing the numbers:
1,88 (largest)
1,8
1,75
1,08
1,008 (smallest)

The numbers in descending order are: $\boxed{1,88; 1,8; 1,75; 1,08; 1,008}$.

3.1.2
Compare 0,9 and 0,900.
Adding zeros to the end of a decimal number does not change its value.

$$0,9 = 0,90 = 0,900$$

Therefore, $0,9 \boxed{=} 0,900$.

3.2
Convert $\frac{2}{5}$ to a decimal fraction.
To convert a fraction to a decimal, divide the numerator by the denominator.

$$\frac{2}{5} = 2 \div 5 = 0,4$$

The decimal fraction is $\boxed{0,4}$.

3.3
Round off 74,153 to two decimal places.
The first two decimal places are 15. The digit in the third decimal place is 3.
Since 3 is less than 5, we round down (keep the second decimal place as it is).

$$74,153 \approx 74,15$$

Rounded to two decimal places, the number is $\boxed{74,15}$.

3.4 a)
Calculate $2,5 + 16,23$.
Align the decimal points and add.

$$2,50$$ $$+ 16,23$$ $$\overline{18,73}$$

The sum is $\boxed{18,73}$.

3.4 b)
Calculate $9,51 - (2,1 + 1,05)$.

$$9,51 - 3,15$$ $$9,51$$ $$- 3,15$$ $$\overline{6,36}$$

The result is $\boxed{6,36}$.