How is the Zakat calculated on money that my mother deposited in an Islamic bank, upon which a year has passed, knowing that this money increased and decreased?

Zakat is a pillar of Islam, and it becomes obligatory on wealth once it reaches the nisab (minimum threshold) and a full year has passed over it. It is not permissible to delay its payment. Whoever delays it must repent and seek forgiveness, and pay the zakat for every past year by estimating the wealth and giving one-quarter of a tenth of it. If the wealth falls below the nisab, there is no zakat due on it for that year. Whoever delays zakat is sinful and must repent and hasten to pay it for all past years. Nothing of it is waived, while striving for accuracy in its estimation.