ANSWER:

You have to pay Zakat for the whole amount and this is every year. 

Shaeikh, say in 2016 one had 100K in the bank and paid Zakat, 2.5% and in 2017 now is 120K, shall one pay ZAKAT for the whole sum or for the 20 extra

Zakat | January 24, 2018 4:08 pm
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I am fron Bangladesh. Here in Bangladesh, madrasa (school for islamic teaching) are mostly run by peoples donation and mostly the orphan and the child from poor background go there. Can i donate my Jakat money to a madrasa or it must be given to the poor peoples hand.