A simple calculation to determine the No. page in the Koran

Quickly through No. part :
If we ask which page will start in Part (GOZ`A) 9 for example?
Our process is simple : Part 9 of so number = nine
nine minus 1 = eight
eight X 2 = 16
then add two figure to the right of No. 16,
then 162 this is the No. Page, which will start by Part IX
repeat to you another example of the 21 Part (Goz`a) , 21 minus 1 = 20 = 2 x 20 = 40 add two right-hand figure becomes 402.