An operation is commutative if changing the order of operands does not change the result.
Division of whole numbers is not commutative. $\frac{a}{b}\ne \frac{b}{a}$

For example: a = 2, b = 3; $\frac{2}{3}\ne \frac{3}{2}$

Let the number be $y$.

$(y-\frac{1}{2})\times \left(\frac{1}{2}\right)=\left(\frac{3}{8}\right)$

$(y-\frac{1}{2})=\left(\frac{3}{4}\right)$

$y=\frac{3}{4}+\frac{1}{2}=\frac{3}{4}+\frac{2}{4}=\frac{5}{4}=1.25$

Raju's total monthly income = ₹ 34000

Raju’s expenses on food = $\frac{1}{4}\times 34000$

                                         = ₹ 8500

Money left with Raju = 34000 - 8500
                                   = ₹ 25500

House rent = $\frac{3}{10}\times 25500$ 
                   = ₹ 7650

$\text{Now, money left with Raju =}25500–7650$ 
                                                = ₹ 17850

Children's education = $\frac{5}{21}\times 17850$
                                   = ₹ 4250

Money left with Raju = 17850 – 4250
                                   = ₹13600

Let the  total number of pages in the book be $x$.
 $(\frac{4}{9}\times x)\text{pages are read}$

$\text{Remaining pages}=x–\frac{4x}{9}$
$=\frac{5x}{9}=30$$\Rightarrow 5x=30\times 9$
$x=54\text{pages}$