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?

2.3 Step 1: Calculate the total fraction of salary spent on rent and petrol. $Total spent = \frac{1}{2} + \frac{2}{5}$ To add these fractions, find a common denominator, which is 10. $\frac{1}{2} + \frac{2}{5} = \frac{1 \times 5}{2 \times 5} + \frac{2 \times 2}{5 \times 2} = \frac{5}{10} + \frac{4}{10} = \frac{5+4}{10} = \frac{9}{10}$ Step 2: Calculate the fraction of salary left. The total salary is represented by 1. $Fraction left = 1 - \frac{9}{10} = \frac{10}{10} - \frac{9}{10} = \frac{1}{10}$ 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, $0,9 = 0,900$.

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 $9,51 - (2,1 + 1,05)$. Step 1: Perform the addition inside the parentheses first. $2,1 + 1,05 = 3,15$ Step 2: Perform the subtraction. $9,51 - 3,15$ $9,51$ $- 3,15$ $\overline{6,36}$ The result is $\boxed{6,36}$.