(i) Step 1 : choose bigger number : 2520 >405 
On dividing 2520 by 405 , we get
quotient = 6 , remainder = 90
$\Rightarrow$ $2520=(405\times 6)+90$

Step 2 : On dividing 405 by 90 , we get
Quotient =4 and remanider= 45
$\Rightarrow$ $405=90\times 4+45$

Step 3: On dividing 90 by 45 , we get
quotient = 2 and remainder = 0 
$\Rightarrow$ $90=45\times 2+0$


Step 4 :  Since remainder is zero, stop the process
therefore , HCF of $405$ and $2520$ is $45.$