All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.
The easiest way to calculate your Zakah (obligatory charity) is to select a given month in the year for this. That month should be the month in which you usually have the Nisaab (Minimum amount of property liable for the payment of Zakah). For you, that is March. You should thus give your Zakah for March 2000.
The Zakah should be on all the money you had at that specific date i.e. 47,250. The calculation of the Zakah of this amount is as follows: 47,250 x 2.5% = 1181.25 In March 2001, you give the Zakah in the same way, be it less or more. So you do not consider the 2nd or 5th or 6th, 7th, 8th, 9th or 10th month, etc – your Zakah is only due in March every year and on the money you have at that time.
Allaah Knows best.