We can create a critical number while evaluating the figures when calculating annual interest for an account like General provident Fund. Popular method is cubersome and time consuming. So, if a more simple formula found it will make such task simple and so we can leverage time we sparing thereby. Traditionally we adopt the method of calculating progressive total for every month and thereafter we arrive the average of its Grand total and finally we calculate its percentage. The traditional method is illustrated in TABLE-1.

Suppose, think about a scenario where subscription and refund is the same through out the year and no withdrawal. Now for the time being forget about Opening Balance (we can take up it later). Devide the Grand total in table-2 with the constant subscription. Then we gets the crtical number or the magic number 78. 78 is true when you apply any number in such a scenario as subscription.
In the traditional method part of opening balance in grand total is equal to twelve times of the the same amount. For example, when grand total of table-2 is substrcted from the grand total appearing in table-1 we gets an amount equal to twelth times of opening balance that appeared in table-1, i.e. 876000-156000=60000*12=720000. With these facts we can adopt a new strategy for doing interest calculation. Remember the scenario, no change in suscription
and no break, no withdrawal too.

So, the new magical formula is Interst = (2000*78)+(60000*12)=876000/12=73000*8%=5840,
In another format Interst = ((2000*78)/12)+60000=73000*8%=5840. See Table-3.

Subscription increased from 2000 to 3000 from may onwards and no withdrawal. Here, a new critical number is needed. In these circumstances 2000 is repeating through out the year. Whereas, 1000 is repeating through eleventh month from may i.e. may through march next year(See TABLE-4). So, devide the Grand Total in TABLE-4 with 1000, then we gets the critical number 66.

New formula here is (see TABLE-5): INTEREST =(2000*78)+1000*66)+10000*12)=942000/12=78500*8%=6280.
In another format Interest = (((2000*78)+(1000*66))/12)+60000=78500*8%=6280(See Table-5).

We can create critical numbers for each change in subscription. Such numbers are recorded below:

MONTH            CRITICAL NUMBER

• APR                     78
• MAY                    66
• JUN                     55
• JUL                     45
• AUG                   36
• SEP                   28
• OCT                   21
• NOV                  15
• DEC                  10
• JAN                    6
• FEB                   3
• MAR                  1