Which $of$ the following situations require the number $of$ payments $(n)tobe$ calculated $as$ a central step?

Multiple select question.

Determining the time required for monthly payments $of$ $$1900to$ pay off $a$ $$250,000$ mortgage.

Determining how long a trust fund $of$ $$100,000$ can sustain quarterly withdrawals $of$ $$2000.$

Determining how much will $be$ accumulated $if$ $$1000is$ saved every quarter for $20$ years.

Determine the loan amount $if$ $$2500is$ paid semiannually for $3$ years.