Is zakat obligatory on an amount of EGP 20,200, which reached this nisab in Rabīʿ al-Ākhir 1434, knowing that its origin was EGP 2,500 deposited since Rajab 1427, and I was unaware of the obligation of zakat on the nisab of silver? And how can I calculate it?

Zakat becomes obligatory on wealth once it reaches the nisab (threshold) of gold or silver. If the questioner has not paid Zakat due to her belief that it was not obligatory, then she must investigate the time when her wealth reached the nisab. She should calculate the Zakat due on her wealth after the completion of each Hijri year and pay all that is due, because Zakat does not lapse with the passage of time. If she is unable to ascertain the exact amount, she should pay what she believes most likely absolves her responsibility.